Measure for the bottom door. I build the bottom door out of 2"x2" beam for the top and 1"x2" beams for the rest of the door. I used a thicker top beam because this door is heavier, and I thought it could use more strength at the hinges.
When measuring, make sure you put a tile down on the floor, if you have not already tiled it. Leave about 1/8" gap at the bottom of the door to give the door room to swing open without scraping the floor. Also leave about 1/4" inch gap for the sides of the door (that is 1/8" for each side). The top and bottom beams should be the width of the door, and the side beams will go between them. On my project, the top and bottom beams were 23.75", and the side beams were 19 1/8". Still, take your own measurements.
Attach the door by using clamps again to hold it in place while attaching a pair of small hinges. Keep the screws for the hinges in the center of the hinge holes, or you may have trouble opening or closing the door.
You can go ahead and put chicken wire on this door now, using the same method as for the top door.
Test the door to make sure it opens and closes freely.
Install latches. I put latches on either side of the door so the cats cannot bend a side of the door because it has no latch.