Ryan basically described the process although it's not as bad as you may think.
First, I had it easy, I didn't have to draw the floorplan. I had some PDF files from the architect who worked on our renovation projects over the years. I wasted a lot of time cleaning them up to get them down to just the basic - really all you need are the exterior walls and partition walls. I say I wasted because I later found another PDF they had provided that had layers. Acrobat Pro recognizes the layers and provided buttons to allow me to turn layers off. That allowed me to generate a cleaned up version in just a few seconds.
If you aren't so lucky, I think you could put together what you need with most any sketching package. Since AutoCAD effectively eliminated my favorite CAD package, Generic CAD, I have used something called TinyCAD. I would suggest that you don't need anything close to a perfect scale drawing - you just need it close enough that the rooms look like the rooms. It is kind of cool to have it scale - that's a lot of work.
The other place I wasted a lot of time was scaling the grid. I think I rescaled it four or five times trying to get the granularity I wanted. Once you get used to doing it, you can do it pretty fast.
I disagree that the sliders on the small icons are useless. They are actually quite useful. When you click on the round button, a dialog box pops up with the percentage and you can easily adjust it even though you don't have a slider to move. Surprisingly you can't get pretty specific with your percentage values even with the tiny bit of travel of the slider.
Clearly this doesn't work on a small form factor like a phone but that was not the intention. This was done as an experiment and I have since thought of several other things I want to try. I am also going to write up a separate thread of suggestions based on what I have learned so far from this experiment.