Button is signed by taking your thumb and index finger and tapping them together, while the middle, ring, and pinkie fingers are outstretched. In other words, in ASL, the 'button' sign starts out as the letter 'F'. Once you have your thumb and index finger touching, you then tap them to center of your shirt or dress to indicate where buttons usually are. You can also indicate a particular button by making the sign in the location of the button you are referring to. For example, to indicate a sleeve button, you would make the sign around your sleeve.
Older toddlers love learning how to use buttons, and the exercise is great for the development of their hand-eye coordination. We use the sign button, so that baby can ask you when she wants help fastening the buttons.
We use and teach the sign for button in association with dressing up. We also teach the sign when working on sensorial development and using a blanket that has buttons to feel, count, and identify colors.