This guide assumes that you know the very basics of modding with MultiMC so if you don't understand them then please go read my previous forum post found here:
A short list of things that I will be covering;
Selecting the appropriate Java version
Setting up Memory usage
Unstable Forge versions
Trusted sites to get mods from
Editing configs for mods
Dealing with crash reports
So without Further ado let's dig in shall we. Java is the core thing necessary to run Minecraft. Every mod is developed using java, but some people may have trouble distinguishing 1 version from another, which can ultimately lead to errors, crashes, and strange bugs such as using too much RAM, or CPU. So with that said the general rule of thumb is to use the most up to date x64 version of Java, unless a mod specifically states otherwise. The most up to date version of x64 java can ALWAYS be found here http://www.oracle.com/technetwork/java/javase/downloads/jre8-downloads-2133155.html
However downloading and having it installed is not the end of this. MultiMC is very similar to Mod Organizer in the fact that you can individually customize various instances, including the java version that you want, all the way to setting memory usage, and even java arguments, however I will not be covering java arguments in this tutorial because they can vary wildly and are not necessary for this particular tutorial.
Moving on you have now downloaded the most up to date version of x64 java, so how do you select it?
First you need to right click the instance you wish to mod, or create a new one and click "Instance Settings" or select the instance and press the button of the same name to the right of the window, this will open this window;
On this screen you check the box beside "Java Installation" then click the "Auto-Detect" button which after a few seconds should bring up a list of all installed versions of Java. it should look something like this;
In my example I only have 1 version of java installed, but the way that java works is that there is a 'default' installation, which is represented by a star by its name, if this version is not the most up to date x64 Java version then you need to select it, and hit the "OK" button.
Next in the previous window you need to check the box beside "Memory" this is so that you can set up how much RAM you would like this instance of Minecraft to use. It should look a little something like this;
The things you should know about Memory usage is that too much ram available can cause just as many problems as too little, and on a side note the "PermGen" is now obsolete with java 8 so it is unnecessary to set this for instances using Java 8. A general rule of thumb is that you will rarely need more than 3072MB (3GB of ram) for the maximum amount of memory, the Minimum however is up to personal preference however it cannot be higher than the maximum. However if you have a lower end PC you should not set it to use more RAM than you actually have. (Side note 1024MB = 1GB of ram you can just use a calculator to multiply it to find how much ram you want to use)
Next up is unstable forge versions. First off all the most stable version of Forge is, is what the authors of Forge have tested, and recommend. However given the early state of Modding in 1.10.2 this does not however mean that higher versions are likely to cause more problems in your game, and often times can mean that many mods being developed use newer versions of Forge. Once there is a consensus on a Forge version in the mod development community it will be more important to select the build of forge but for now we will go with the most recent build.
First off in the same window as the "Instance Settings" you will look to the left and at the very top of the list there will be something that says "Version" click on that and it will take you to a different section that will look like this;
On the right you will notice a button Labeled "Install Forge" click it and it will bring up a new window listing many versions of forge for this version of minecraft;
Scrolling to the top will reveal a version with some sort of bug beside it, this bug however does not mean that it is a "buggy" version of forge it is only meant to represent the latest release of forge, at the time of writing this tutorial it is Forge 126.96.36.1995, so select this version of forge (which may cause problems for other mods but we will get to that later)
With your forge selected in the previous window again below the "Install Forge" button you will see another labeled "Install LiteLoader". LiteLoader is used for several GUI mods, but is not necessary for all mods, generally you do not need to install it unless a mod specifies that it requires it, but if it does it is the same installation process as forge.
With the necessary requirements we can get down to downloading mods, and installing them, however before installing them there are a few things that I would like to go over given a small problem that I have seen on the previous post, Please be careful where you download your mods from!
Trusted sites for mod downloads go as following;
https://mods.curse.com/mc-mods/minecraft - The Curse main site is the most secure, and trusted source for mods it is heavily moderated, and harmful mods are always removed
https://minecraft.curseforge.com/mc-mods - The CusrseForge site is just the mod development site for curse.com, it has alpha, and beta releases and gets updated before curse.com
http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods - The Minecraft Forum is also heavily moderated, however mod devs will link to their personal sites there.
Mod Devs OFFICIAL Sites - Lastly are the mod developers official sites, as listed in The Minecraft Forum, other mod sites tend to either be scams to infect your computer with malware, or they steal developers work, and put it up for their personal gain using ad.fly, or some other form of ad revenue.
Given that I covered how to install mods in the previous tutorial if you do not know how I will point you to the link at the top of this post, and ask you to go read it, and we can move on to configs. Before you can edit your configs you will need to run the instance once so that any mods you are using generate their configs, once the instance has booted up you can just close out ouf minecraft and go back to the main screen of MultiMC. On this screen you will Right Click the instance and select "Config Folder" and it should bring up a window with the directory being something like "instances/-instancename-/minecraft/config" and in this folder you will most likely see something along the lines of ModName.cfg now while I personally prefer Notepad++ any text editing program is sufficient for editing configs, (IE: Notepad, gedit, or TextEdit are defaults for each operating system) you will need to open these files with one of those programs.
For the sake of this tutorial I have chosen to use Tinker's Construct, which can be found at https://mods.curse.com/mc-mods/minecraft/tinkers-construct, and has Mantle as a requirement which is found at https://mods.curse.com/mc-mods/minecraft/mantle. The config for Tinker's will be named tconstruct.cfg and look like this when opened;
Now that might seem a bit intimidating at first glance, however I promise that it is fairly easy to deal with, but a few general rules when dealing with configs;
They are CASE SENSITIVE
They should only be edited when Minecraft is CLOSED
You should always follow suggestions in the configs.
With that out of the way we can start editing this config, and I will focus on 4 different things in this particular config;
Dimensions that slime islands can spawn in.
In cases like this you will need to be able to look up a Dimension ID. for the default dimensions -1 = The Nether, 0 = The Overworld, and 1 = The End. This is a blacklist meaning that any dimension here will not have Slime Islands spawning in it, so lets make it to where Slime Islands will spawn in the End, but not in the Overworld for a few laughs.
As you can see I removed 1 (The Dimension ID for The End) and replaced it with 0 (The Dimension ID for The Overworld), now it is not necessary to have the same amount of dimensions in the list as previously, but I wanted to make the point that adding and removing work as well, now if you boot up the game with those changes you will find no slime islands on the overworld, but they will spawn in the end.
The Ore to Ingot Ratio in Smeltrys.
As the # mentions this can be set to any number, except it cannot go below 1. Since the config states that we cannot do that then we wont do that instead we will set it to 3.5 which will look like this;
What this will do is when you melt down an ore such as Iron Ore, in the smeltry instead of giving 2 ingots worth of fluid (244mb of Molten Iron) It will now give 3 and a half ingots worth (504mb of Molten Iron). After saving it and booting up the instance you just have to build a smeltry and throw in some sort of ore and it will show 3 ingots, 4 nuggets, and 8mb if you hover over the the melted product in the smeltry GUI.
Whether all tool parts can be crafted in the part builder table.
Typically metal tool parts must be crafted by pouring molten metals into a cast from the smeltry or some other source containing said metals, by changing this option we can make it a little easier for our selves to get that special Tinker's Iron Pickaxe that we want, so all that you have to do is change that "false" to a "true";
This is where what I said about case sensitivity comes in, if you were to put "True" or "TRUE" in there rather than "true" the config may not take and revert to "false" when the instance is booted up to test this change.
Ore Generation per chunk.
Now you may be trying to figure out what this might mean, the simple explanation is that with these settings Tinker's tries to spawn 16 cobalt and 16 ardite ore per chunk in the nether. A chunk is a 16x16 sqare going from height limit to bedrock, so if we were to change these numbers we could end up with several different results. Since these are whole numbers however we must keep them as such (no decimal points)
With these Tinkers will now attempt to spawn 32 Cobalt Ore per chunk and 8 Ardite Ore per chunk, so we should find significantly more Cobalt compared to before, and significantly less Ardite.
With that done that covers configs, and we can move on to everyones headaches Crash Reports.
Typically crash reports are because YOU have most likely done something wrong, and don't take that personally I've had hundreds of them my self from simple little mistakes, and often times it can be from something as simple as a typo in a config, or adding in 2 mods that have code conflicts, and I am going to teach you how to differentiate between when you make a mistake, and when a mod dev has.
First you need to find the crash that has occurred, thankfully Minecraft puts them in a convenient folder named Crash Reports located in "instances/-instancename-/minecraft/crash-reports/" They are also Time Stamped meaning that if you crash at 3:30 PM or 15:30 on October 16th of this year it will be something like "crash-2016-10-16_15.30.44-client" meaning you can accurately find the crash if you have more than one. Next up you open up the crash report and try to identify the problem. Generally speaking you will find the problem in the Stacktrace which will look a little something like this;
Now this is several lines below the start of the crashlog, but the easiest way to locate it is to look for the line "A detailed walkthrough of the error, its code path and all known details is as follows:" This particular crash is cause by NEI, it has a problem with players starting in a different dimension than the overworld, but the line that identifies NEI as the problem is "at codechicken.nei.ServerHandler.dimChangeEvent(ServerHandler.java:157)"
If you are ever particularly stumped on what mod is causing and issue google can be an invaluable tool, so you can try searching 1 or more lines from a crash logs Stracktrace to attempt to find information on a particular crash, and often times it will give you a solution.
And with that you now should be able to mod your game with relative success, I wish you luck and thanks for reading!