Nice coastlines in GIMP

[Apa1994]

Pretty new to this, have gone through a few tutorials but I have one question:

How do I easily create nice, random-looking coastlines where I still have control of were the "line" goes.

[Gidde]

One thing I do sometimes is to randomly generate it using either the tutorials or an experimental method, then I put a white layer underneath it, reduce the opacity on the black and white landmass layer that I randomly generated, and then make a new transparent layer on top and actually draw the coastline in. That way I have more control, but I have a starting point that's inherently random.