Well idk if it’s the best method but…

You should first draw your map in black or white, the sea just has to be a different color than the land, then fill the land in whatever color you have, then get a couple of NASA blue marble images, and paste it in over the land. If you don’t like the image you used, delete it or put another image over it ( and add a layer) and use an airbrush eraser to see if you like it now.

Also I recommended to use layers for every image until your done ig.

Im not that good at explaining but I hope you understand a little