For those familiar with the basics of GIMP there is a quick way to create a custom, decent looking, world map. Like this one: http://opengameart.org/content/fantasy-world-map-0

Here it goes:

1. Launch GIMP.

2. Open this file, created by yours truly: http://opengameart.org/content/sheet-of-old-paper-0

You can use other images as the base, but for this tutorial it will do. Remove all the layers containing text, leaving only the old-paper texture. Lets call this layer "paper".

3. Create one more layer, transparent. Call it "ocean". Fill it with solid blue-ish color of your liking (e.g. 367aba ).  

4. Right-click on "paper" and select "Alpha to selection". This will select the shape of the paper. Decrease the selection size, moving away from the borders of the paper image: (menu) Select->Shrink->40 pixels (other amounts are possible for a different size of the paper image). Finally, invert the selection by pressing Ctrl-I or (menu) Select->Invert.

5. Select the "ocean" layer. Delete the selection. Now you have a blue ocean and some distance from the borders of the paper image. Invert the selection again (Ctrl-I). Unselect everything, (menu) Select-None.

6. Create a new layer, called "landmass". Fill it with solid white. Generate "Difference Clouds" via (menu) Filters->Render->Clouds->Difference Clouds. You can play with the parameters, but a good starting point will be: Size X and Y = 4, Details at 8. Play with the "Randomize" option as well. Some practice is necessary to get the feeling of what you will need.

7. With the "landmass" layer selected and filled with white-grey-black noise-like clouds, go to (menu)-Colors->Posterize and set the number of colors to 2. This will create a black and white image that looks like black and white map.We are getting there.

8. Lets make a mask, by removing, say, white color (you can remove black if you like white part of the map to represent continents). Go to (menu) Colors->Color to Alpha. Choose the white color to be removed and press ok.

9. Change the color the "landmass". Use Colorize option in (menu)->Colorize and set Hue/Saturation/Lighness to you liking. Suggestion: H=45, S = 39, L=46.

10. Duplicate this layer, call "landmassOutline". We will use it to add nice border for the continents.

Right-click on "landmassOutline" layer and select "Alpha to Selection". Shrink the selection by 2 pixels. See above, step 4. Delete the selection, Del. Deselect everything, (menu)->Select->None. Make the "landmassOutline" a little bit darker, (menu)->Color->Brightness and Contrast, decrease Brightness to -100. Merge this layer down to "landmass".

11. Lets cut the part of the landmass outside the ocean. Select the "ocean" layer, Right-click->Alpha to selection. Invert the selection, Ctrl-I. Click on the "landmass" layer and delete the selection. Invert the selection again and select None.

12. Optional. Add some noise to both the "landmass" and the "ocean". (menu)->Filters->Noise->HSV noise. Play with the parameters.

13. Add a layer, on top of the rest, called "grid". Create 64x64 pixels grid lines, using (menu)->Filters->Render->Pattern->Grid. Set the "grid" layer mode to "Overlay" and decrease the effect strenght (Opacity) to 20. To remove the part of the grid that goes outside the paper texture, right-click to select the paper texture and alpha-to-select it. Invert the selection with Ctrl-I, click on the "grid" layer and delete the selection. Re-invert the selection, Ctrl-I, and then Select None.

14. Add a layer on top of the "ocean" but under the "landmass". Call it "screen". Set the foreground color and back ground color to Black and White. Fill the "screen" layer with radial shape, gradient of "Foreground to transparency". In fact, we need transparency to foreground, so select a "<->" arrow next the the gradient mode. Click in the center of the map, hold the lef mouse button and drag it to the lower right corner (or any other corned). Release to create a radially-shaped gradient. Again, we want the "screen" to be of the size of the ocean, so "Alpha to selection" in the ocean layer, invert the section (Ctrl-I), click the "screen" layer and delete the selection. Re-invert (ctrl-I) the section, then (menu)->Select None.

15. Beautification: create a separate, 64 by 64 pixels, transparent image. Use white pencil of 1 px size to draw three waves of approximately the same shape. Too crest per 64 pixels. Waves must be vertically uniformly separated.Make the image "tileable" using (menu)->Filters->Map->Make Seamless.

Select all. Add a layer on top of the "ocean", under "screen" and call it "waves". Fill it with pattern from the clipboard. To avoid painting over the ocean size, you may select ocean (alpha-to-selection on the ocean layer) to restring the area you are going to fill with waves. Select None.

16. Finally, set the following modes for the layers:

"ocean" -- Overlay, Opacity = 55

"waves" -- Soft light, Opacity = 55

"screen" -- Normal, Opacity = 40

"landmass" -- Value, Opacity = 100

17. Play with the layers' modes and the opacities values to get the desired look.The suggestions given above satisfy me, but you taste and need may differ.

Thats it. I hope it took under 15 minutes. Did it?

PS: I'd love to see your results.

YD.