Well, this is pretty simple. The video shows you everything you need in a dramatic visual fashion. hooray.
here's the transcript, though
Take a condom--unlubricated is better, but all my roomies had were lubricated ones and those worked OK, albeit a little slippery-ly. Stretch it out over the microphone and down past the end of the mic to the cable.
Use some duct tape to tape the end of the condom to the mic cable and seal it off.