Tie your hair into a ponytail. Doughnut buns look good high or low, so the height of your ponytail is up to you. Be sure to smooth out the hair on your head, and make the ponytail tight.
Use a fine tooth comb to help smooth out your hair into the elastic. If you prefer a looser, messier look, then do not pull your hair back as tight.
Consider spraying your hair with some hair spray to help hold your hair in place, and smooth it down.