Microsoft Visio is a good tool to design modern day floorplans you can use in your game for a simple reason: Real people and real companies use it to draw some real floorplans. So it is a "realstic" approach to use this tool, and build on all the options you can get with this. But your job at making a map with Visio is much harder than hitting the New button, and selecting a Floorplan template.
This time we will start with a small office. At first we should use Page Setup from the file menu, to set drawing scale. I would set up for something like 1:60. It should be good for miniatures. Also take time to set up drawing size to some size you can use. I set it up for A3 size myself. Then it is time to set up something like a Grid, which can be done in Tools menu. And of course we can set grid to fixed spacing with 5' spacing and 0 offset.
Now from the shapes, select the Walls, Shell and Structure (US Units) stencil, and drag and drop an exterior wall to your drawing area. If you select the wall (that is: Left click on it with a normal arrow cursor), you will see two little green squares. You can drag these squares to move the ends of the wall (it can make wall shorter, longer, can rotate it, etc.).