First my garage floor was very uneven and heavily sloped with a fall of around 5" over the 16' span. Apparently, builders start by building the garage so that they can mix more concrete inside the garage for the build of the main house - that seems a reasonable explanation to me when I tried to comprehend the large lumps of concrete stuck to my garage floor. So, I took the advice from GLWB and bought two large sleepers to lay on the floor, which I bolted to the floor and took out some of the fall in the fall. Then I built out the legs for the side beams. In the interest of having the three beams accurately aligned, I screwed them together and ran them over my planer together in an attempt to have them lined up.
I spent quite a while with my water level. I'd read in John's GLWB where they had used food dye to colour the water to make it more visible, so I decided to add some ink, which had been idling in my office for some years. Yes it makes for a nice visible colour in the tube, but hey does it create quite a mess when you forget to hold both ends level and black ink comes squirting at you, all and sundry! So I abandoned the water level and took out a 6ft spirit level which seemed to work fine. I later checked it with a lazer level and my measurements seemed good. However, it's not a trivial job, certainly for me single-handed nad first timer, I spent quite a few days before I was satisfied that my jig was square and level.
While I had reasonable success in keeping the jig square and level, I did find the side beams warped and so I had to use a combination of clamps and windlasses, with 4' spacers in between to finish with parallel side beams. Also, even though I had taken quite some time to carefully cut the angled timbers which support the centre beam on my mitre saw, I still had some jiggling to do to get a properly aligned centre beam.