Heya. I was going to reply to this the other day but figured someone with more experience than I would get to it first. Our resident expert, if I may be so bold, is Chris West, a professional cartrographer whose credits include D20 Modern Critical Locations.
I've been trying to emulate his style to some degree in this thread (edit: to be clear, those aren't his maps, but they're similar to his style). The basics process I've come up with so far is:
1) Start with a base drawing as a reference. I used an existing map but a sketch or architectural drawing would work just as well. If you want realism you really will want to do your research here - missing a stairwell, bathroom, kitchen, or some other key piece of your building will lead to confusion even if they don't realize why "something feels off".
2) Create a new layer for each thing you're working on; you can flatten them later but, for now, it'll give you flexibility. This works whether you're using a raster app like GIMP or PS or a vector app like Illustrator or Inkscape.
3) Start laying down shapes and use textures, layer styles, and hand-painting to make them look like the surface you're trying to emulate. I started with the parking lot, which was just a big square. I drew the outline, filled it with an appropriate texture, then added some staining and so forth. On a layer above that I drew some line shapes, flattened them into a single layer, then I selected that layer (ctrl-click in PS) and stroked the selection with white until I got something that looked like parking lot markings; with that done I played with blending a bit so they showed some of the background and didn't look so stark white.
4) Repeat step 3 until you have added as much detail as you want.
For walls I used rectangular and oval marquee tools to define the areas and then stroked with black; had to try it a few times to get a line weight that looked good to me. I did this in stages, erasing out the bits I didn't need. You could also do it with line objects. The doors and windows are just rectangles and a stroke layer style. I created one of each size I needed and then dragged (alt-drag to copy a selected area) and rotated them to position. I believe I used a slight drop shadow on the wall layer to "anchor" the walls to the floor. You could get do this a little more precisely if you wanted but I was going for fast.
Creating furniture was a similar process to doing the doors, except that I drew a more complex object. Once I had that object though I could just drag them around to create new ones in different positions and rotations. I put a very simple layer style with a drop shadow in there. With furniture, I kept all items of a like kind on the same layer; and I positioned those layers logically...so table shadows fall on chairs, etc.
These were just experiments, though, and I'm no expert at this stuff. It could all be done more precisely or more roughly, depending on what you're going for. Hopefully some other folks will weigh in on this one. I know I'd love to see some Chris West tutorials some day (wink, wink, nudge, nudge). Until then, hope this helps some.