Measure the height of your scarecrow with a tape measure. Measure from the feet to the shoulders and then the width of the shoulders. Divide the shoulder width by two and note the measurements.
Make seven marks on the 20-foot length of 3/4-inch PVC pipe with a tape measure and marker. Measure and mark four 18-inch pieces, one piece the height of your scarecrow and two pieces that are half the shoulder width each. Cut through the PVC at each mark with a small-tooth saw.
Drill a hole through the center of the four-way cross joint and the center of the 3/4-inch PVC end cap with a 3/8-inch drill bit. The center of the cross joint is the apex where all the ports meet. Flip the cross joint over and drill a hole through the center of the other side with a 7/8-inch drill bit.
Flip the cross joint back over and align the end cap's hole with the hole in the cross joint. Turn a 3/4-inch long, 3/8-inch thread bolt and washer through the drilled holes. Turn the cross joint over and thread a nut onto the bolt threads inside the cross joint. Use a socket wrench to tighten.
Place the cross joint assembly open-cap up. Slide the four 18-inch PVC legs into the four ports on the cross joint. Pop the PVC section that is the height of the scarecrow into the end cap. Fix a 3/4-inch PVC T-joiner onto the top of the height pipe and pop the two shoulder pipes into the open ports of the T-joiner.