Before you begin installing, I recommend creating a backup of .minecraft/bin/minecraft.jar

Installation Using .class Files:

1) Open/Unpack minecraft.jar using an archiver such as 7zip/WinZip/WinRar
2) Delete the folder called META-INF (if it exists)
3) Insert the contents of the "modfiles" folder into the minecraft.jar; overwrite any existing classes.
4) Close/Repack minecraft.jar
5) Run Minecraft