I have been teaching several children and adults to close large buttons using large button squares. These are simply 2 pieces of fabric -one with a button sewn on it and the other with a slit to button onto it.
Some of my clients are ready to work on closing smaller buttons before tackling the ones on their clothes. So I took some of my old blouses that are bit too tight on me and decided to donate them. I wrapped them over a couple of layers of cardboard to make them more firm while the individuals closes first the buttons to the blue blouse and then the buttons on the red one. I think it is easier to learn this skill when the buttons are in front of you rather than on your own clothing where you can't look directly at them.