Here's my simple guide:
1) Download skin
2) Unzip it
3) Upload the files to your server (follow instructions in skin package as to where. Placement varies but is usually inside the imges folder)
4) Go to your admincp
5) Go to
Styles & Templates -> Download / Upload Styles -> Import Style XML File
6) Find the xml file and click import
7) Enjoy
Hope this helps, if you run into any problems just report back