Tileset Creation 101
Tileset Creation 101, or TC101, was an old site maintained by Ice M A N containing a fair amount of tileset creation tutorials. The site was hosted on nagcentral.com. Among the article subjects were masking, creating/converting palettes, textured backgrounds and compiling tilesets. The site was shut down and parts of the site were eventually merged into jcsREF, a general JCS resource.
Nowadays, with the site shut down and only part of it left in jcsREF, Disguise's Tileset Resource is usually considered the place to go for tileset help.