Since the back would go against the wall, I had to start with the front side down and attach the back rails first.
I hit on the idea of using concrete blocks both to square up the assembly and to space the slide members. That meant carrying 36 blocks down to the basement. Was I sore afterward! I haven't brought them back up the stairs yet. I wouldn't recommend this method, and I certainly wouldn't do it again. It worked, but the blocks are just too heavy.
Here is the bench with the back rails attached.
Then I had to roll the bench upright. I managed to do it alone, but it would have been much easier with a strong helper.
Then I squared up each slide member and screwed it to the lower front rail. That held it square while I attached the upper front rail.