Step 0: If you haven't already done so, update Minecraft to 1.2.5. (You'll be prompted on login.)
Step 1: Download the mod here: http://binarymage.com/projects/mystcraft/
Step 2: Extract it using WinRAR (or any similar program).
Win7: Type %appdata% into your search box on the start menu
XP and such: Start->Run %appdata%
Go to .minecraft, then bin
Open minecraft.jar with WinRAR (or whatever you use).
Step 4: Dump the entire contents of the Mystcraft folder (the contents, not the folder itself) into the jar.
Step 5: Inside the jar is a folder named META-INF. Delete it.
That's it. You're good to go.