By hand. Take a needle, with waxed thread about 8" to 12" long, and thread it up from the inside, through a button hole. With a toothpick for spacing, go down the diagonal hole. Go through the same two diagonal holes 4 or 5 times, then switch to the other diagonal. After another 4 or so loops around that one, take the toothpick out, thread the needle up from the back, under the button, and wrap it around 5 or so times to strengthen and space it. Tie it around, thread back down under, tie it off, trim. Repeat for the rest of the buttons, and.....