Okay, let's finish the walls and colour the rock around before we start putting in some furniture.

Finish laying down the lines for the wall outline as above so that your walls are a closed loop of lines, or until the lines reach the edge of the page. This will be important in a minute. You should have something that looks something like this:
map1.jpg

Now it's quite hard to tell where the wall is and where the corridors are (okay, not too hard on this map, but on a larger map this would be tricky). We need to flood fill the wall areas to make this clearer. To do this pick the fuzzy select tool:
select.jpg

Make sure that sample merged is off. Now click within an area you want to fill. You should get a large area highlighted with a line of black and white 'marching ants' that should encompass the area you want to fill. If instead you get one square selected, then you've got the background layer selected rather than the Walls layer. Go to the layers dialogue and click the wals layer and try again. Now as we used the brush tool, our walls have a slightly soft edge to them. This means that the fuzzy select region doesn't quite go up to the wall lines. If we fill at this point then we'll get a thin line of white between the wall and the fill. So go to Select>Grow... and pick 2 pixels. This expands the selection to the middle of the wall lines.

Now go to Edit->Fill with foreground colour, or ctrl-, This should give you something like this:
map2.jpg

This is starting to look a little better. Finish filling the walls in the same way.

Now we want to lay some internal walls that don't take up full 5' squares. Do this by switching back to the paintbrush tool and using the click, press-shift-and-drag, click method above to lay down internal walls. Don't worry about doors, we'll do that next.

Remember - if something goes wrong you can always hit ctrl-z to undo it.

This should give you something like this:
map3.jpg