As promised, I'll show you how I cover the front of a locket with polymer clay.
I flatten the clay already using the pasta machine so that I'll get a smooth and flat base that I can easily work on. It's hard if you make a ball and then flatten it on top of the locket. It usually gives you a bumpy surface not as smooth.
As for the thickness of your clay..it depends on the design that you're going to make. If you are going to do some stamping, I suggest that you use a thicker sheet but if you're just going to use it as a base, you can just use a thin one.
Always make sure there are no air bubbles trapped between the clay and the locket. You can remove it by lifting the clay sheet a bit (the part where the bubble is present) and then flatten it again pushing the air bubble out. :)
Here are the videos!:)
When I don't have a specific cutter that I can use for other locket shapes...I do this technique:
Hope these videos will help you in some way! Have a good week!:)