Step1: Install the mod from wherever it comes from once it has completed DO NOT EXTRACT just save it to the desktop
Step2: Find your .minecraft file on you computer XP: go into run and type %appdata% then click on roaming then on bin and it should be in that folder Linux: Go into your home folder then click view, show hidden files then find it inside your home folder
Step3: once in the .minecraft folder click on bin then open the minecraft.jar folder with: XP:winrar / Linux:Archive manager
Step4:open up the .zip mod and copy all the .class files into minecraft.jar
Step5: Finally delete the META-INF folder because that folder will stop the mod from working.
Congratulations you have now installed your mod to minecraft and will be able to use it in the game.