So, adding a WCS is not actually all that difficult. But in my experience finding the process clearly explained is more challenging. I finally found a process that works well for me (with thanks going to Kyle Johnston, U. Waterloo, for outlining the process). Also, there are probably multiple ways to do this, but here is my attempt to lay it out clearly. I hope it helps!
(for a more formatted version, try this pdf file)
Steps for Adding a WCS to your FITS image: