On inside corners like that, it is often easier to put down a strip in the corner--just about an inch wide or so. Then, cover the horizontal and vertical stabs up to (and overlap a little) the strip in the corner. This is just about the easiest way to do any inside corner. Also, if you are bound and determined to do it all with one piece of covering, you cand use a small trim iron, and get the heat down into the corner. Then, chase the iron with a cool, damp rag to cool the film back off--keeping it from shrinking back out of the corner.
Hope this helps...