1. Learn the concept of non-linear time and optionally quantum mechanics.
2. Download MinecraftCoderPack(MCP)
3. Download a vanilla minecraft jar
4. Unzip the MCP and put it's contents in a folder
5. Copy paste the "bin" and "resources" folder from your ".minecraft" folder into the "jars" folder of your unziped MCP
6. Run decompile.bat (takes ~10 minutes)
7. Download / install eclipse
8. Run eclipse and when it prompts you for a workspace navigate the the "eclipse" folder from your unziped MCP folder.
9. (You now have all the source files in eclipse in the package explorer) Look around the source and mess with it.
10. Finish playing with the source (making your mod, go look up tutorials for that)
11. Run "recompile.bat" then "reobfuscare.bat"
12. Give out the contents of your "reob" folder from your unziped MCP. It contains your modded files.
13. Claim internet fame for your modding efforts