I don't use PS (more of a gimper), but it sounds right to me. You should have a black and white image of the land mass (full white land and all the rest black). Unless you have other info on your base map, that should be about all you need it for. If there are other things on the sketch, just bring the background layer up above the landmass one and carry on... I would suggest lowering the opacity of the background layer and locking it. This way you will still be able to see what is underneath, and have no fear of accidentally adding to or removing from it.

I hope that helps...or is at least not sending you on a wild goose chase....