This method takes a few hours longer than doing it the following way:
1. As said in guide, remove wow (unless you want to keep retail)
2. Get WoTLK disc or download it from
World of Warcraft Community Site
3. Install WotLK, make sure you don't let it autopatch.
4. Start patching from 3.2
This will save you around half of the patches.
Ps. if you want a 2nd copy so you can keep playing retail, make sure you delete (but backup!) the registry files of the retail version.
Ps2: should be in Emulator guides I'd say.