Adding a mod?

how exactly do u add mods to minecraft on windows 7 OS i put all the class files into the jar and deleted the meta inf b4 i did that but they just dont seem to work v.v ne ideas?

@ReAp3rCl0Wn

Some mods don't work with eachother, and some need modloader to work. Old mods to new client versions etc.

Which mods are you using? What client version?