How to make a tileset in eight greatly compacted steps.
1: Draw the tileset (don't you hate it that this is only one step?).
2: Save the tileset image as a seperate file, with an M or something added onto the filename.
3: In this new image (which is your Mask) remove everything which you do not want to be solid from the tileset. Also any blank stuff which you want to be solid, make some other, non background color.
4: Copy (unless they're already there) both images into the Tiles subdirectory of your Jazz2 folder.
5: Open JCS.
6: Click the "Set" button, the "Add" button, and fill in the fields. If your tileset does not have a JJ2 compatible palette, select the "Remap Tileset Palette" button.
7: Click "Ok". Now find your tileset in the White list of tilesets, select it, and click the "Build" button.
8: Click "Ok", click "Ok" again, find your tileset in the Pink list of tilesets, and open it.