Hi Eilathen,

I have to say, I just fake it and add smooth & wiggly bits until it seems about right. One way you can do it also is to take real world coastline silhouettes and invert, resize, stretch, cut up and stick together until you have something new, but the coastline should have the right amount of 'wiggliness' since you have used real coastlines to start with.

In terms of overall shapes there are a few tutorials out there on world building, you could look at those. Or you could use a generator like 'Fractal Terrains' . Regional shapes are often defined by natural barriers like rivers and mountains and following those gives them a more natural look.