At this point, I should let you know that besides a handful of small projects (building a bed in a van and an outdoor kitchen at Sturgis) I have zero construction experience. This build has been a challenge for even the most seasoned construction people, and I went into it with blind ambition.
We jumped into the ceiling first, hanging the joists on top of the south wall framing, attaching it to joist hangers on the other side. Surprisingly, they came out relatively level despite not using one instrument to check it.
Then, we discovered what would turn out to be a major issue with the space. The floor was not level at all. Intentionally so. They used to drain the boat that was house backed there on the sloped floor. We noticed the slope when trying to build the first booth wall. We built it square, but had to shim it in to keep it up. Notice how the top of the wall touches the joist on the right, but needs a 2x4 block to stay attached on the left.
Instead of taking them down and leveling the floor, we just went with it. Turned out that every measurement would not match on any corners anywhere. It was also complicated by the fact that we built staggered, double stud walls for maximum sound isolation. We decided to build these walls square, shim them in and attach them to the concrete below. This allowed us some wiggle room to make the walls line up at the appropriate 1" spacing before nailing them in.