Take a look at the pallette file that comes with the program. When you create a bitmap file, you simply copy the colors as your "tile".
1 - Create a bitmap file. (I think it has to be the same dimensions - i.e., 80x80, 100x100, 256x256). *note* It uses the old vanilla Civ3 world size dimenions.
2 - Open the palette that comes with the program.
3 - Using the eyedropper tool in a paint program, select the color (tile) that you want, and start painting!
Just to note - I think the plains forest, and jungle palettes are swapped. So be careful. (not sure if I fixed that when I re-uploaded it).
If you want a different sized world (i.e., 180x100), you can make it 180x180, and fill the bottom 80 pixels with 1 color (i.e. brown for mountains). In the Civ3 Conquests Editor, press F12 to crop the map.