One of the great things about Winamp 2 skins in particular is how simple the format is - basically just a bunch of BMPs in a zip file. Pretty much anyone could make one. You didn't need any special software or code knowledge to inspect an existing skin and see how the pieces fit together. You could make one in MSPaint if that's all you had, as I did for my first couple skins.

(There were also a few INI-style config files, mainly to define colors of dynamic things like text. For most skins you would just need to copy the defaults and maybe change a few color hex codes.)