In it's most basic form, you just convert or create some graphics. Outpost2 will accept normal Windows bitmaps for the tilesets. You just make the bitmap 32 pixels wide, by 32*numTiles pixels high. Pack each 32x32 pixel tile right on top of each other.
Now, that will allows the game to load and use the graphics, but there is still more to having a tile set that works correctly. Unfortunately, we've never provided editing for the other misc details to handle things smoothly (yet). So I guess you can just not worry about them right now.
If you're wondering what the other misc details are, think what happens when you bulldoze a region, or lay down tubes or walls. It has to change the tile graphics from whatever it was to the bulldozed, tubed, or walled tile. There is a table that is supposed to handle this translation (and is built into the map files that use the tilesets). But alas, editing that table is something that's still only planned for the future. In the meantime, you can use the table from an existing map, and organize your tileset so certain things correspond, like the bulldozed, tubed, and walled tiles. Lava tiles are probably also important. (Just make sure the indexes are all at the same position). Anyways, editing for that will eventually become available, so even if you have no idea what I'm talking about, you don't need to worry too much.
So yeah, just made a big tall bitmap with the tiles you want in it.
Poll