you need to take a properset ( a jelly gun) then instert in the side of the donut, the pull the triger on the properset until the donut is filled. then take some extra dough to close in the hole! in...
They usually use a frosting bag with an attachment on the end to pump the jelly into the center of the doughnut. If you look closely at the side of the doughnut, you'll see the hole where the jelly...