I would build this helmet in the following order:

1. Cut out the plates.
2. Finish the plate edges and corners.
3. Shape the front and back plates for the top and rivet them together.
4. Trace the opening in the top of the helm from the inside and add an inch all around. That is the patten for the cap plate on the top of helm.
5. Cut the cap plate and finish the edges.
6. Dish the cap plate and fit it to the opening in the top of the helm (on the inside). Rivet the cap plate into place.
7. Shape the back plate to fit the helmet top and rivet it on.
8. Punch/Drill the breath holes on the face plate.
9. Fit the face plate to the helm and rivet it on.
10. Polish the helmet and the brass plates for the cross. (I use 3M surface conditioning disks)
11. Rivet the arms of the brass cross over the eye slots.
12. Rivet on the main part of the brass cross. Be sure to use a steel reinforcement strip under the part of the cross that goes over the eye slot (between the eyes).
13. Repolish around the cross if needed.