A good technique is to clamp a hand (of power) drill in your vice and hold the pegbox in our other hand. Drill slowly constantly checking on the position of the drill bit.
Note that the walls are getting thicker towards the floor of the pegbox, see the dashed red line. The front wall, forming the extension of the nut "b", runs at an angle of approximately 105 degrees, "x" in Fig. 2. The bottom of the pegbox should be of consistent thickness at the same time allowing sufficient space for the strings, placing it normally about 4 mm, "t", below the pegs, see dashed line defining the bottom in Fig. 2.