Hi Abu,
I have built a couple of Ziroli designs. The crutch is the "backbone" of the plane. Ziroli usually makes this crutch stand on a number of building blocks. The crutch usually has cross braces too since this might also be banana shaped. The crutch is now laid on the building blocks and later the formers are passed through with the crutch being located on the inside part of the fuselage.
The formers are then aligned according to plan and then stringers are glued to the formers with the added advantage of fine tuning the position of each former according to plan.
It is a simple but an effective method of making a construction of a fuselage.

Hope this helps.

Reuben