Creating Classic Skins
From Winamp Developer Wiki
Creating a Classic Skin: Introduction → The Base Skin → Paint the Main Window → Paint the Equalizer Window → Paint the Playlist Window → Paint the Minibrowser Window → Paint the AVS Window → Paint the Winamp 2.9/5.x Windows → Create Custom Cursors → Edit the Configuration Files → Compress to .WSZ format → Submit to Winamp.com
Description of a Classic Skin
Classic skins are skins that adhere to the Winamp 2 style model. Classic skins are easier to create than Modern skins but do not allow the author to change the form or function of the player. Classic authors may only replace a standard set of images that alter the player's visual appearance.
A lot of people get really excited when they find out that they can change the look of Winamp through skinning. The problem is they get turned off once they hit a roadblock, be it what to do next, hit a problem submitting to Winamp.com, or indeed, just figuring out how to get started. So here we have a set of simple steps that holds your hand from initial idea to finished masterpiece. Note: Cool kids still use (and create) classic skins. Good luck!