detach your symbol box for easy access. Its good to zoom in to about two grid boxes. (shortcut: SPACEBAR + CTRL to zoom in quickly, SPACEBAR + CTRL + ALT to zoom out quickly)
Now start creating the symbols you will need for the map. Its fairly simple process, so I don't feel much of a need to go into each one step by step. Just go from Grid square to square and put out a symbol in each one.
For doors, use a box with a white fill, to cover any lines you put them over.
For your rubble, use the pencil tool with a blue fill and no select. This allows you to quickly draw rubble in. (pencil too: hold down ALT after you started your line, before you end your line and pick up your stylus, this will auto connect the lines.)
Once you have completed all the symbols you think you will need, take your selection tool and grab each one, dragging it to the symbol box and naming each one (if you desire) and you are done with this layer, lock it and hide it and move on to the next layer