The spar butt fitting holes were marked out then drilled on the drill press. I made sure the drill press table was in-square to the drill bit.
Before threading through the ribs onto the spars, I marked reference points on the bench where the spars would be using the end of the bench as a straight line reference. Ribs were then threaded through. I then screwed down some L blocks to the bench along the reference points, which were then eventually clamped to the spars to keep everything dead straight and flat on the 3/4 shims I used under the spars so the ribs had clearance off the bench top.
The bench is dead square, this enables me to use the end of the bench as a square reference by screwing on a stop [basically a lip on the end]. The spars butt up to the stop, then you know everything is square without having to constantly bring a square into play.
The three compression strut ribs were then glued into place, the other ribs left loose for the time being [these need to be free-moving so you have drill clearance when drilling for 'drag wires at the ribs stations just glued]. Some builders like to use masking tape to hold the rib during curing, I like to clamp on a butt block to the side that's set up on the rib pencil line, then press 'home' the rib to it, clamping on a scrap piece on the other side which holds the rib against the butt block.
After curing, I dry fitted the spar butt fittings and drew a pencil line around them, then removed them. The st.2 1/4 rib spar apertures were then widened to accommodate the 1/4 ply on the spars with a jigsaw. Feeding in the st.2 1/4 rib I then marked where the spar butt fittings will clash, removed the rib and notched the apertures with a hacksaw blade then jigsaw. On the front spar, the clevis pin bracket that mounts to the spar butt fitting also clashes