Upholstery suppliers sell uncovered button forms that come in two separate pieces. This allows each upholstery job to have perfectly matched buttons, because the upholsterer makes the button covers from the original fabric used to cover the furniture or car seat. A professional button covering machine can be used, or a cheaper hand held button press can be found in a fabric store. Either tool provides good results as long as the fabric is properly cut and tucked into the button forms.
Cut a fabric circle using either the cutter that came with the button press or the scissors and cardboard tracing circle that comes with the handheld button covering tool. Trim off any loose threads that are left after cutting the circles.
If you are using a professional button press, place the fabric circle face up on the top section of the button die, and put the button shell face under it with the curved side facing up into the press. Push the shell up so it fits snugly in the die. You can use your fingertip, a pencil, or anything that fits inside of the shell. Push the excess fabric inward tucking it into the shell.
If you are using a handheld button tool, set the top die on a flat surface, centre the fabric circle face down over it, and put the shell upside down on top of it. Press the shell and fabric down into the die, and tuck the excess fabric in.
Put the piece called the button back into the bottom of the button die. Place the bottom die on the bottom of the press, or if you are using the handheld tool set it on a solid surface.
If you are working with a press, pull the press handle back and down, closing the dies together, and apply solid pressure to seal the pieces together. If you are using a handheld button tool, put the loaded top die on top of the loaded bottom die and tap the die pieces together with the hammer.