Generally, you make the map like you make any RPG map but keep lite on the moveable bits. So no monsters, no characters, dont draw anything on the map the players might want to move about. Then you import the map as a base layer and put all the stuff you can move on top of it. So doors that can be opened, chests, items they pick up, plus all the monsters are icons/tokens that you add. Generally a VTT map does not have a grid on it and you let the VTT app put one on for you.