See https://www.cartographersguild.com/a...chmentid=80066 for an excessively long-winded step-by-step example.

The simplest way that I have found to get good a good balance between quality of results and time of execution results is to start at a much lower resolution than your final result (say 1/16 of your existing map or around 480 pixels wide), process the resolution, scale up the result by a factor of 2, reload your mask and force your terrain to match the coastlines of the now higher-resolution mask, process that resolution, scale it up, and so on until you get to your desired resolution. If you want to go one step past that and then use GIMP to scale back down that final step, you can get clean edges on coastlines and rivers.

For best results with masks in Wilbur, I do recommend looking at using two-color GIFs as the inputs to Wilbur rather than other file formats.