Please note that I do not use wonderflex to any extent. Tried it, didn't like it. That being said, these suggestions might not work due to the nature of wonderflex itself.
The armband pieces at first glance look like a big complicated project, however, its not really all that bad.
What I would do is first sew (or buy) a simple armband that is tight enough to stay on your upper arm. Then I would line the inside of the armband with lines of hotglue, or something to that effect.
Once the armband(s) are prepped, fit, and ready, you can start building up the fancy part.
The next few steps should be able to easily be done in wonderflex.
Cut two triangles out of craft foam. Be sure they are close to the same size.
On the top triangle, trace out the intricate gold pattern.
Carefully cut out the pattern from only the triangle you traced the pattern on.
Now glue down that down to the first triangle.
Depending on how thick what you are using is, you may have to do more than just the one layer of design.
Now, with the armband on your arm carefully bend the cut triangle w/ design to the shape of your arm. Go easy and slow, it will want to go back to flat, so you may have to shape/curve the piece a few times before its just right.
There are a few ways you can attach the piece to the armband.
Hotglue, Sewing right through the piece & armband, and any other ways you can come up with. Be sure you attach the piece and armband before painting.
Use your favorite kind of paint to paint, detail, and finish the piece.
The best advice I can give you is to be patient. It may take you a few trys to get it to look "right", but thats how this kind of thing works.
~Yuni's the name, Prop's & Armor are my game.~