Giide's "Ascension's Atlas Tutorial in GIMP" land glow layer wont blur right
So I tried using the Ascension's tutorial in GIMP. And for some reason or another, I always get stuck at the land glow layer step (for reference, its step 3). Every time I try to do that particular layer, the guassian blur will always blur into the landmass instead of outwards into the water, meaning that the land glow effect doesn't work when I move the layer down. I got it to work one of the times but every time since I haven't been able to reproduce it. (also that map got corrupted when my laptop battery died...).
So these are the steps that I follow:
1) Make a new transparent layer called "landglow"
2) Fill it with light blue (64,200,255)
3) Add a layer mask from the coastline channel
4) Click on the thumbnail that shows up next to the layer
5) Apply the gaussian blur at 22px
6) Move the layer down below land but above ocean
7) Flip my table cause it didn't work
Any ideas what I did wrong?