You need a rectangle of fabric, about 5" x 5" (that works for my standard sized hen- you can adjust for your bird). I make mine out of outdoor fabric, two layers sewn together to make it more sturdy. Attach two ties across two opposite sides, center the ties so equal amounts hang off each end. I make the ties about 25" long and then trim once it's fitted. It goes around the crop with the ties horizontal across the crop area. Make sure the lower edge is tight enough that the legs won't get tangled. I have one bird that high steps in her crop bra and I learned the hard way about getting tangled legs, having the straps horizontal, rather than vertical makes it easier to prevent legs getting stuck, I found. Lower tie gets tied behind the wings, upper tie gets tied in front of the wings. Supervise a LOT until you know you have it adjusted correctly. It may take some readjusting to get it right. The bird may walk backwards at first, sometimes they just get weirded out until they adjust, and they may get attacked. I use a twill type ribbon for the ties, and once it's adjusted and on I trim the extra and wrap the knot with co wrap (so no possibility of someone getting a string from picking at it). I have also seen them made with velcro straps. Here is a link where someone made one entirely out of co wrap/vet wrap:
https://www.backyardchickens.com/threads/pendulous-crop-but-otherwise-seems-healthy.808714/
I will get a picture of my girl in hers and post in a bit. She has a pendulous crop, and has done really well since putting the crop bra on. She was losing weight, and is now putting weight back on and the crop empties well.
****Edited to update my measurements, I've made several, went back and measured the one in use now.
