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
Skinning The AVS Window
This file is layed out much like the other Winamp bitmaps. Once again, the image is divided into various segments only divided by spaces between the sections. The first component in the image, in the upper left-hand corner, is the close button in a pressed state. This close button when pressed will close the AVS window as well as turn off the plug-in.
The next graphic, directly to the right of it, is the upper left most section of the AVS title bar. Be sure not to allow any text to go past the end of the allowed graphic size due to the fact that it will be displayed incorrectly.
Directly to the left of that graphic is the graphic of the title bar which is used to tile horizontally. This graphic should be symmetrical due to the fact that as the AVS window is stretched horizontally, this image is continually duplicated to give the illusion of a stretching title bar.
At the upper rightmost section of the image is the close button for the AVS window in an un-pressed state.
The next row of images contains two really long columns followed by three flat row graphics. The leftmost column is the left border of the AVS window followed by the right column being the right border of the AVS window. The next graphic is the leftmost section that composes the bottom of the AVS window. To the right of it is a little section which is tiled horizontally as the window stretches, just like its title bar counterpart. The final graphic, the rightmost graphic of the second row, is the rightmost section which completes the bottom of the AVS window.
That wasn't too tough, was it? Not at all! This should probably take no more than an hour or so to skin and creates an all around fuller affect if skinned with the same "scheme" of the Winamp interface.