First you need an art program like Photoshop, I use Photoshop "Elements" that came with my computer. It's just a cheaper edition of the program with a few less features.
After that, the main tip I can give is use LOTS of layers. Icy Inlet had 11 layers, many of which were almost transparent. Sea Storm had 6. The point is, don't try to make you map with just one or two layers, it's too easy to make mistakes.
If you really want you can download the icy inlet psd file from my website:
Don't forget the file is 3.6mb
Oh yeah, sponge, I'll do some betatesting if you want