Determine where the buttons will be placed on your cushion. If you are making a floor pillow and just want one in the middle, measure from each top corner diagonally. Where the lines intersect will be the mid-point of your cushion. Mark your button location with fabric marker on both sides of the cushion.
Thread your long needle with a length of sturdy cord. Pull the ends even so you have a double cord. Be sure your double cord is long enough to reach through the cushion twice with a few inches to spare.
Tie the first button to the cord ends. Push the needle through the cushion at your marked point until the button rests on the surface of the fabric and the needle comes through the other side of the cushion, where you have marked it.
Run the needle and cord through the shank of the second, matching button, and run the needle back up through the cushion--beginning on the mark where your needle just came through and coming out just under the first button. Pull up the thread so the second button is secure. If you wish for a “deep button” look, pull the cord until the the buttons reach the desired depth. Run the thread through the shank of the first button and tie it off securely, clipping the threads and tucking them out of sight beneath the button.