1) Take a floor flange and place it on the non-cut flat surface as shown in the bottom of the figure.
2) Use a pencil to mark the hole locations.
3) Fasten the wedge to your drill press (or some other flat surface) so that the top surface is horizontal - you'll have to prop up the thin edge of the wedge. Make sure it's secure - you don't want it to go spinning off across the work area.