Please back up your minecraft.bin file and your save files before installing. Installing the texture pack -shouldn't- screw anything up but back up just incase you have problems. Don't whine to me if you don't.
1) If this is your first time installing an HD(Anything over 16x16) texture then follow the first video
2) If you've ran the HD fixer atleast once before and simply want to change out your texture without having to delete your bin folder, follow the second video
First Time Installation: http://www.youtube.com/watch?v=MkKGebwGv4Y
Replacing a Texture: http://www.youtube.com/watch?v=4fGyqbhaEZw