Hi Krabby,
If for the Alpha TE, it's wonderfully simple. On the file menu, select Make Map and follow the prompts, then find the map.bmp in your project folder. Optionally modify it with your favorite graphics program, and drop it into the texsrc folder.
If it's for the current TE, the procedure is similar except that you must limit the bit depth to indexed (256) color.
If you want to contact me next week, we can make you into an expert on all this in about two hours.