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Mark a 45° angle on the 4x4 post. Cut the post along the marked angle. Cut the opposite side of the post to finish incomplete cuts, if necessary. Drill a hole in the flat bottom of the post. Mark entry holes on the four sides of the post. Drill side entry holes upward on a 45° angle.

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Lay your post flat on its long side. Use your square to draw a 45° angle from one corner of the post until the line ends at the opposite side edge. The angle, when cut, will form two edges about 7 in (17.8 cm) long and two that are about 4 in (10.2 cm) long. Position your post somewhere it will be safe to cut with a saw. For example, you might clamp the unmarked edge to a workbench or scrap wood so you can cut the marked edge more easily. Cut the post along the marked angle with your circular saw.  Use caution when operating a saw. Improper usage can result in serious injury. Always wear safety glasses and a face mask. Substitute a hand saw for a circular saw. However, cutting with a hand saw will take longer and require more effort. The blade of some saws might not extend all the way through the post. In this situation, turn the post over and mark the same angle on the opposite side. Cut the angle on the opposite side to finish the incomplete cut. Orient the flat bottom of your post, which is opposite the angled top, upwards. Use a tape measure and a pencil to mark the center of the flat bottom of the post. Use a 7/8-inch bit to drill a 4-inch (10.2 cm) deep hole at this center mark.  Drill the hole on a straight up and down angle with respect to the flat bottom of the carpenter bee trap. If you're a poor estimator of distance, measure the length of your drill bit and note the point it is about 4 in (10.2 cm) long. Drill to this point. Each of the four sides will need a hole to make your trap most efficient. Use a tape measure and pencil to mark one hole on each side. Each hole should be 2 in (5 cm) from the bottom edge and ¾ in (1.9 cm) from side edges. Hold your square beside the hole to help you judge the angle. 45° is halfway between horizontal (flat) and vertical (up and down). Position your drill parallel to this angle and drill upward until the hole connects with the hole drilled in the bottom.  Drill holes in this fashion at each of the marked points on the four sides of your post. All holes should connect with the straight hole drilled in the center of the post's flat bottom. The angle for your side entry holes doesn't have to be perfect. For quick, easy, and accurate angled holes, use a pocket jig as a guide. Pocket jigs can be bought at hardware stores.