First, designate the side boards to match with the end of the top and bottom boards as shown in Step 7 (A-A, B-B, C-C, D-D). The top and bottom boards will become the template for the drill holes in the side boards.
Align boards and clamp to hold in place. Use scrap blocks to prevent damaging the centerpiece.
Drill a hole through the existing hole and into the side piece. The depth of the hole through both pieces combined should be 1-1/4".