Create an image that is 320 pixels wide by a multiple of 32 pixels high. NOTE A: For better looking tilesets, have it be 256 colors, using a JJ2 compatible palette.
Fill the tileset up with the tiles you want to have, in various ways, but leave the top left tile empty (a tile is 32x32 pixels).
Unless you wish every non-transparent pixel to be solid, create a mask. 0,0 color pixels the player can walk through, everything else is solid. Make sure the mask and main image are the exact same size.
Copy both images into the subfolder "tiles" of your JJ2 directory, go into JCS, click "set", then "add".
Tileset name: What you want your tileset to be called.
Filename: Filename for your tileset.
Image: Select the main image.
Mask: If you made a mask, select the mask image here. Otherwise check the "Automask" box.
NOTE: If your palette was JJ2 compatible, do NOT check "remap tileset palette". Otherwise, do.
Press ok, select your tileset on the list of tilesets, and click the "build" button. Press ok, ok, then view your tileset in JCS.