Mark and cut four 16-inch lengths from the ABS pipe. (The lengths will function both as sleeves for the cabana legs and depth guides for the holes.) Wrap a piece of masking tape around each pipe sleeve, 12 inches from bottom end.
Using post-hole digger, dig one 12-inch-deep hole for the right front leg, and place a pipe sleeve in the center (see photo). Check that it is completely vertical, using level, then backfill to hold upright.
Tip: To soften extra-hard ground (like clay soil), start digging the hole, then fill it with water. Let soak overnight and start digging again.