Measure 36 inches across the entrance of the walkway with a tape measure. Align the measurement so the start and end points are centered with the width of the walkway. Mark the end points of measurement in the soil with the blade of a post hole digger.
Dig a 24-inch-deep hole at each measurement mark on each side of the walkway entrance with a post hole digger. Fill the bottom of each hole with 6 inches of gravel.
Lower one end of each post into the two holes. Fill the holes with the soil that you removed from the hole and pack it down around the posts with your feet.
Center one 1-by-4-by-60-inch board between the posts on the front and back sides. The top edges of the boards should be flush with the top ends of the posts. Orient the boards horizontally. Drive four evenly spaced stainless steel wood screws through the boards into the posts at each joint with a screw gun. These are the horizontal supports.
Position one 2-by-2-by-16-inch post across the horizontal supports between the posts. These are the cross supports boards. Place one 6 inches from the inside of each post and one at 18 inches. Drive two screws through each cross support board into the horizontal supports at each joint with a screw gun.