i just downloaded this just to run a test for you..so better no make a mistake applying this..hehe
dont worry..it's just this simple..
1 download the file
2 extract the rar file..
3 after extracting..u should see a bunch of files like these..

4 double click the file "skin pack".. the one with a different icon..
5 this will install / extract some of the skins for ur ym8..
6 if u aren't contented with that..then u can double click on the other skin files..this will be individually installed to ur machine..
7 right now im using the "Yahoo! Messenger 8 Black Skin" file name..
8 go to ur ym..sign-in and click on messanger..
9 in the middle there's an option there for changing the skin..
10 u will see ur additional skins and u can now apply it..
11 dont forget to say tnx..