I'm in the process of making some maps for my GURPS fantasy campaign and, being the lazy person I am, I'm ripping the map style from a map of Tamriel, the continent from the Elder Scrolls games. The map can be found here : http://www.uesp.net/wiki/File:TamrielMap.jpg

My question is, how do I make coastline with the kind of "frills" seen on this map?

I am a bit of newbie and using GIMP.