[Trerro]

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).

Step 3:
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.

[Sonya the fox]

Modifying directions:
Step 0: you can't just update to 1.2.5 anymore.  If you don't already have a copy of the jar, I don't know what to say to help you, as Minecraft.net is now 1.3.2

Step 1:
Obtain Minecraft Forge: http://jenkins.minecraftforge.net:7070/job/Forge/152/
Obtain "Mystcraft 0.9.1.02 Forge 152" from the Mystcraft site as normal.

Strp 2: apply to Forge and Mystcraft separately into their own folders

Step 4: Drop Forge in first, THEN Mystcraft