Please? How do I use a skin

Hi guys! Very new to WP. I have looked at Alex’s page and the results of the template competition, awesome work. My only problem is throught all the documentation and support, I can’t find how to use one of them. I really like Human Condition, I dl’d it, but how do I use the darn things????
A really detailed answer would be great.. Pretend I am 5 years old. LOL
Thanks in advance