Each bulkhead is clamped to one of the lateral 2x4s. A string is run end to end down the centreline of the jig. Each bulkhead is aligned using a plumb bob to make sure it is not twisted, and it is positioned laterally by making sure that the plumb bob line just touches the centreline. The vertical position is set by measuring the height of the top of bulkhead from the 2x4.

Read on for more.

Here is an overall view of the bulkheads in the jig. The fuselage is built upside down.

The firewall is on the left. The next bulkhead is a massive one that has the main wing spar carry through structure. The green stuff in the middle is the seat rib assembly

Here is a closer view of the front part of the fuselage.