Step 1: Download all 6 prereq mods and patches, as well as IC2 itself. To make this simple (and to ensure we can get the stuff when the IC site is overloaded), I have packaged all of this together on the CC server at:
http://chaoscluster.com/misccrap/IC2andPrereqs.rar.
Step 1 1/2: If you don't have WinRAR for some reason, grab it from it rarlab.com. If you're not on Windows, get something else that handles .rar files, I don't know offhand what the common choices for Mac and Linux are for this, but will add that to this step if anyone knows.
Step 2: Extract that archive you just downloaded.
Step 3: Start with a FRESH MC install. If you want to make absolutely sure you have a fresh copy, do the following (but make sure you read the important note first):
A. Start->Run->%appdata% (or %appdata% in the search box or Windows Explorer).
B. Delete .minecraft
C. Run the game and wait for it to redownload everything.
D. Make a world and connect.
IMPORTANT: This will, of course, completely delete and reinstall the game. If you're trying to save anything like single player worlds you like, be sure to back them up first.
Step 4: You'll notice the prereqs are lettered from A through E so there's no question of what order to do this in. Repeat the following process for EACH of these mods.
A. Go to %appdata% (Start->run->%appdata% or type %appdata% in the 7 search box.
B. Go to .minecraft/bin, and open minecraft.jar with WinRAR.
C. Open the folder for the mod you are installing. Copy the contents of this folder (not the folder itself) into the jar.
D. If you see a folder named META-INF in the jar, delete it.
E. Run the game, connect to the world, and play for a good few seconds to ensure whatever needs to run actually ran.
F. Close the game.
Again, repeat this process, separately, for EACH of the premods, in order.
Step 5: As step 4, but for that 1 loose class file that isn't in a folder.
Step 6: Go to %appdata%/minecraft. You'll notice there is now a folder in there called mods. Drop the IC2 jar into that folder (don't extract it or anything, just copy it as is).
Step 7: Run the game, and connect to your single player world again.
Step 8: Test to make sure it's working properly:
-The game should not take noticeably longer to start up. If it's taking forever, something failed.
-You should have sound.
-You should be able to make a tree tap on a crafting table, using the following (P is a plank, X is a blank square):
XPX
PPP
PXX
Step 9: You can now join the server. Enjoy.
