I've been wanting to do this for a while but never made the time. Here's a video of my process for making an 'old blue' map.
I made a couple of mistakes on the video, notably setting the recording framerate to "ass", but there you go - better luck next time.
My process for making these is completely different from my hand-drawn maps, which should be obvious immediately. The Photoshop template I'm using has a bunch of useful layer effects all ready to go.
Instead of drawing walls, I'm painting floor space. I'm literally just grabbing basic shapes for brushes and painting in white, and the outlines grid and get added automatically.
It's a shame I didn't manage to record my mouse cursor, as I think that would make it all a bit clearer - instead of rooms appearing, you'd see me dragging a square brush around for corridors and rectangles, and then using circles and hexagons to make the various notches.
What's fun about this is that I can build up a complex outline a bit at a time (see 1:30 to 2:30) instead of knowing in advance what the outline of the room is going to be.
Architectural adornments like this don't do anything for grid-based fights, but I hope they convey an baroque impression that would affect the GM's description of the place. Room #6 is clearly not a crude stone room.
The texture I lay down from 7:40 to 8:15 isn't lost, I just hide it and bring parts back with a chunky 'boulder' brush. (You can then see me go around twice with a 'pebble' brush.)