Welcome to CC honeygirl1. It depends on what you are comfortable using. There is a tutorial on this site to make a royal icing tiara. I'm getting ready to do my first one, but I am going to roll gumpaste into ropes and form it over a pattern, because it is for a 2 yr old.
I made my first one from the tutorial on this site and it turned out awesome. But i would make more so you have a backup just incase one breaks.
Most Tiaras are made from plastic. If you warm the tiara in an oven at about 150 degrees, you can press it flat. Then you can press the tiara in Silicone Plastique that has been rolled out twice as thick as the tiara. Wait about an hour and you will have a flexible silicone press mold.
Hope This Helps,