I first made a pattern on cardboard. I never use corrugated cardboard, I get sheets of this stuff from work that is like cereal box cardboard, only much thicker. It is much easier to shape into curves smoothly and is great for patterns.
This is the basic Diadem built, I changed the green gem to the oval one in the final picture as it was more accurate. after a few touch ups with some spot putty, it was ready for paint.