Step 4: Build the Base - Step 3
Obtain the piece from Build the Base - Step 1. Onto one end, thread one of the pieces you made in BtB - Step 2. Now you should have what looks like an "L". Do the same with the other piece from Step 2, but do it on the other end. You should now have that looks like a "C", a "U", or a lowercase "N", depending on how you look at it. I saw a swan feeding a rabbit. What would Freud say?
Just as a bit of finishing, grab the two caps and put them on the free ends of the 18" lengths. The addition of the caps not only adds some finality to the look, but also keeps everything on the level - each fitting has a lip on it and if the caps were not put on, the front would be a hair lower than the back when the stand is built. Plus, small rodents can't get inside to create tiny, tiny rodent cities.
Oh, by the way, now that you have "cheater bars" on each end of the 10" pipes, it should be pretty easy to tighten the 10 inchers into the tee and two elbows. Just use the leverage provided ever so graciously by the 18" lengths to crank down and tighten things up. Don't worry about tightening the 18" pieces into the elbows. Those won't have rotational stress on them and will be as effective as ever with just a hand tightening.