This will work with any version of vBulletin, and will work similarly to other forums like PHPBB and IPB.

Things you will need:
vBulletin installed on your server
FTP Access
Admin Control Panel Access
A new skin/style

Ok, now let's get started.

1. Extract your new skin files into a folder named what ever the skins name is. (Ex: Unzip arcticfresh.rar into a folder called arcticfresh)

2. Go to your Admin CP > Styles and Templates > Download/Upload Styles
- Select the yourstyleexample.xml file from your skin folder. Click upload.

3. Open your FTP Client and connect to your site with the FTP info your host gave you when you first signed up for your hosting plan.
Username: blah
Password: blahblah

4. Navigate to the appropriate folder on your site (look in instructions that came with skin. Usually skins are placed in the /styles/ folder in the forums/images/ folder.)

Upload your skin.

Done! Give it a few minutes to upload, then change your defualt skin in your Admin CP.