Each mod has a specific way to install it. Usually mods come with installation instructions. Read those if they are avalible. If they are not avalible, then open minecraft.jar by using a program like winRar and put the files into the right places (.class files go into .minecraft/bin/minecraft.jar, .lang files go in .minecraft/bin/minecraft.jar/lang, and so on). When installing mods make sure to delete the META-INF folder in minecraft.jar.
Private Mod Note
Rollback Post to RevisionRollBack
I have stopped making Minecraft mods...
but I am now making actual games on my official website http://www.chaomoon.com