Step 1: make your way to minecraft.jar (on most computers its just called minecraft) it is located in your bin folder in .minecraft (since i don't know about anything else i will just post instructions for windows computers) to easily get to the minecraft.jar hold your windows key and push the "r" key a window will pop up in it type "%appdata%" then hit "ok" then go into .minecraft then bin then minecraft.jar.
Step 2: open minecraft.jar in an archiveing program like WinRAR or 7zip.
Step 3: open the .zip folder you should get from downloading this mod
Step 4: download modloader and open that then copy and paste everything inside into your minecraft.jar (clicking and dragging also works)
Step 5: do the same with everything in the zip file you get from this mod
Step 6: Delete the folder META-inf (if you haven't already)
Step 7: (most important step) Enjoy the game and the mod :)

NOTE: this will not work if you have a mod that adds extra armor to the game