Place the pipe on a work surface. Measure 3, 9 and 15 inches from one end of the pipe using a tape measure. Mark each measurement with a permanent marker.
Draw a 2-inch-wide notch at each measurement mark on the pipe with a permanent marker. The notch should slope in to the center of the pipe in a downward direction so that the railing will rest securely in the notch.
Saw each notch in the pipe using a band saw. Carefully guide the saw along the marker lines to complete each cut.
Drill a 1/8-inch-diameter hole through the side opposite the notches in the center of each notch opening using a drill equipped with a 1/8-inch drill bit. These are the mounting holes so that you can secure the notched bracket to the posts in your garden with lag screws.