I don't know of any other tutorials but you could learn by practicing with the tools that you have in Gimp.
Merging mods is basically copying parts from one map and pasting it as a new layer on another map, then moving the new layer on top, zooming in close and erasing (or cutting out) the portions that you don't want. You can then save the file as a jpg, pop it into your game, and see how it looks. Of course you ALWAYS want to make sure you have a backup copy of the original maps.