*This procedure works on my Mac OSX 10.5 machine running MC 1.5_02. It may or may not work on your machine, so take everything that’s said here with a grain of salt. Neither I nor UMB is responsible if you screw up your computer, all information within is derived from personal experience and not extensive scientific testing, blah blah blah don’t blame us if you mess up.
1. Extensive modifications are going to be made to your minecraft.jar (location: Users/usernamehere/Library/Application Support/minecraft/bin in OSX; C:\Users\usernamehere\appdata\Roaming.minecraft\bin on Windows), so back it up. Back up your saves, too. And your computer. And your house. And your cat. Name it (the .jar, not the cat) something like “minecraft_orig.jar.”
2. Open up your minecraft.jar so you can see the .class files and folders inside. Do this by right clicking it and selecting “open with” and then an unzipping program of your choice. I use Stuffit Expander.
3. Download your mod of choice and unzip it. Here I’ve used Risugami’s ModLoader, since many mods require it. Drag the .class files directly into the minecraft.jar folder. If you have extra folders from the mod, see if there are folders with the same name inside the minecraft.jar folder. If there aren’t, put the folders with the rest of the .class files. If there are, take the files from the mod folders and add them to their respective minecraft.jar folders. OPTIONAL STEP: If the mod has files to go into the ‘resources’ folder, put them there. Look closely at how the resource files are arranged. Depending on the mod, you may need to move files and/or folders into the resources folder or add/replace files/folders to existing folders. It all depends on the mod. Be smart, read the forum thread and the readme.
See, this isn’t so hard! Almost done now.
4. Delete the META-INF folder inside your minecraft.jar. VERY IMPORTANT STEP DO NOT FOREGT TO DO THIS OR YOUR COMPUTER WILL MELT INTO A RADIOACTIVE APOCALYPTIC **** HEAP. Or you’ll get a black screen and Minecraft will crash. But why take the chance?
5. Select all the files inside your updated minecraft.jar folder. Click the first file, scroll down to the last file and click on it while holding shift. Now they’re all selected, right? Good. Now, right click any of the files and click “compress ~600 files”
6. Take the Archive.zip and move it into the bin folder.
7. Rename it “minecraft.jar”
8. Slap yourself on the back and have a bowl of mushroom soup and a cookie, because you, good sir or madam, are finished.
Theoretically, this method should work for all mods. the screenshots were taken on 1.4, but theses steps should be valid all the way until Notch adds a modding API.
If you get a black screen or constant crashing, try going through the steps again, adding the files in different orders, etc. Play with how you’ve installed your offending mod. If still no cigar, look through the forum, and use the power of Google. Still no luck? Now you can contact the mod creator or make a new post in the forum.