This last step will be easier if you can gather up a willing victim but it's completely doable if you have a few bar clamps, a few muscles, and are willing to shed a few drops of blood. I gots me lots of clamps but not nearly as many as that Norm Abrahms. Clamp hog.
19. STAND the end pieces upright.
20. LIFT top piece into place on top of both ends and clamp at least one end to the top. Make sure it's a right angle and all square-ish. It's also very heavy. DUDE, you're lifting four ten foot 2X4s PLUS dowels PLUS screws and it's all dripping in preservative!
21. SCREW in the braces you cut in step 17. I recommend three on top and toenail two in on the lower end.
22. SCREW the top to the side. Note the placement of the three screws in the corner in the above and below pictures.
23. Unclamp and do the other side, again making sure it's square.
24. Time to bolt! DRILL a 1/4" hole on the inside of each corner, about as deep as your 1/4" bit. Slide the washer over the bolt and insert into the hold. Tighten that puppy all the way up.
25. Time to play! Climb up on that thing yourself and have some fun before the kids see you.
You may want to sand down the dowels a bit or you can wait for it to happen naturally.