When it comes to drilling holes for shelf pins or supports, I’ll often wait until the cabinet is assembled. Then I use a ¼" hardboard or plywood template that registers against the bottom of the cabinet. This way, you’re guaranteed that the holes will be aligned for stable shelves that won’t rock or wobble.
To drill the holes in the project above, I used a template for both rows of holes, as you can see at right. I started with a strip 3" wide. After laying out the locations on the centerline, you can drill the holes at the drill press. To use the template, just register the edge against the front or back of the cabinet and drill each hole.