Make sure you glue the top of the former to the top rail & keep the center line on the center line of the plan.
Align BF5 with marks on rails & glue with thin CA keeping BF5 vertical.
Lay fuselage frame on fuselage balsa side & align top edge.
Sand bottom planking even with sides.
Setting wing rib tang position, place a piece of tape on boom side over pin holes up to BF1. Extend BF1 center line onto tape about 1/8". Clamp piece of wood on BF1 that extends above side of boom.
Hold straight edge & remove square. Draw a line onto the tape to BF3 approximately.