Dear IC2 dev team:
I apologise greatly if this breaks a rule or is in the wrong section if so please delete/move.
How to decompile IC2 with MCP
The first stage of modding is decompiling this tutorial will give you the code for IC2.
Prep
Grab a clean minecraft jar and install ModLoader ModLoaderMP and Forge. Test to insure it works.
Get MCP/eclipse
Download and install MCP (out of the scope of this tutorial but should be relatively easy, guides are on the MCP wiki). Get (and setup) Eclipse. Trust me, it makes your life far, far easier.
Get IC2 into that Jar
Download the IC2 Jar and move the contents (the stuff inside the IC2 Jar) into the minecraft jar. Go into the "ic2" subfolder and delete anything with Integration in the name (eg the bcIntegration folders) otherwise it won't decompile properly.
Get Ready
Put the bin folder containing the minecraft jar (now with IC2, ModLoader, etc in it) in the "MCP/jars" folder (the bin folder should contain a couple of other things).
Check
You should have in the "MCP/jars/bin" folder: A folder called "natives", jinput.jar, lwjgl.jar, lwjgl_util.jar, md5s, minecraft.jar (which should have all the mods inside it) and version.
Decompile
Run the decompile script and wait for it to complete you will receive warnings about the missing server jar, the modded client jar and possible a hunk reject or something akin. The final test if any of these caused a major issue should be if their is source code in "MCP\src\minecraft\net\minecraft\src".
Mod
Have fun!
To test your code run the recompile script and then the test game script- if your are using Eclipse just save your files and hit the green play button.