To make ours we used frames from the dollar store (if you're going to make some too - be sure they are wood frames and that you remove the glass), clothes that were in our "donate" bin, and a staple gun.
This happens to be the only picture I got of "the process" of making them (kind of hard to hold a camera and staple gun). I would just fit the item of clothing over the frame, staple it directly to the wood then cut off the excess fabric.
Our finished dressing frames were made from clothes that had zippers, snaps, buttons and buckles.
I decided to make our frames double sided (as our play/learning space is small). I also like this idea/tutorial on how to make one frame with removable fabric sets.
I have heard mixed reviews from Montessori moms and teachers on dressing frames/amount of use they actually get - so am happy with my makeshift versions that cost $2 to make, and take up a small amount of space.
You can watch the proper way to present dressing frames on the InfoMontessori channel.