I have an angora rabbitry and wanted to allow them access to the grass beside the cages and the ability to stretch their legs. I used to pick them up out of their cages and physically bring them to the enclosure, quickly shutting the gate behind me. I was sure there was a safer method so made them a habitrail (or rather a rabbitrail) instead.
Step 1: Make a door
it's difficult to see from the picture, but I had to make a door on the end of each of the cages. I cut a piece of wire grid big enough for them to exit to serve as the door and cut the hole in the cage slightly smaller than the door. Secured the door with wire at the bottom to serve as a hinge. The doors are secured at the top with dog clips, and their water dish has two hooks on both corners to further secure the door in the event that one of the clips isn't properly secured.