The guide is a notched block of wood with two holes in it. Each hole has a metal bushing in it to protect the jig from being damaged by a drill bit.
Mark a 90-degree angle on a 2x4 for the notch. Use a band saw to cut the notch, then use a table saw to remove excess length from the 2x4.
Mark the center of each the side of the jig where you are going to drill the holes.
Use a drill press to drill the holes through the jig (Image 1). Use a bit the same size as the outside diameter of the bushings.
Turn the block over and drill a hole through the other side using the same bit. Carefully align the opposing holes, then fit the bushings into holes (Image 2).
Clamp the guide to a mitered corner (Image 1), then drill through the hole in the jig (Image 2).
Remove the clamp and insert dowels into the holes (Image 3).
Cut the extra material from the dowels. Round off the edges of the corner if desired.