You are doing the right thing by posting your trial and error pieces. I am not familiar with the style you are working with, nor the tutorial that you got it form, so I'm afraid I can't offer up much in the way of advice, however I'm sure others will be. In any case, the best way to learn to use GIMP is simply to read up on the software and then mess around with it. Using this tutorial for a few trial runs should help you become more familiar with the software, and with some map-making techniques. I'd also encourage you to try out a couple other tutorials so that you can learn other techniques as well. Keep up the good work man, and here's a bit of rep to set you on your way.

Cheers,
-Arsheesh