Door Openings
Figure out how big you want your openings; mine are 9 inces tall by 5.5 inches wide. Within this space, cut out the horizontal bars on the inside edge of the vertical supports.
Doors
Cut doors from the one of the remaining half shelves, making them slightly larger than the opening so that the door overlaps the opening a bit. Cut on the outside edge of the supports so the door is a bit wider than the opening, and make the door up to a couple of inches taller than the opening.
Note
I cut one quite low down on the cage planning on it being a ramp in and out of the cage when open. I'd recommend checking the height of your plastic bottom before doing this -- I didn't, and my door doesn't open as far as I would have liked it to, though it is still functional.