How to build an obelisk
Set a circular saw to make a cut 1½ inches deep, and cut the line at the top of the shoulder. Then set the saw to full depth and cut the other (tapered) line, taking care not to cut too far.
Flip the post over and cut from the other side, then finish the cut using a handsaw. Cut the other side of the tongue in the same way.
Position the legs against the post tongue and test to see that the leg bottoms are fairly close to 22 inches apart; adjust the angles if needed.
Drill pilot holes and attach the legs to the post top with 2-inch screws.
Drill pilot holes and screw the spreaders to the legs. You may need to flex the legs a little to make the spreaders fit.
Drill countersunk pilot holes and screw the crosspieces to the legs with 1¼-inch screws. Weave the verticals over and under the crosspieces. Fasten the verticals to the post and the spreaders with 1¼-inch screws.