There are two things needed for successful doming: 1) A resin with high surface tension, designed to work on photos, papers, and other flat materials. and 2) A doming tray, which elevates and supports your cut-out shapes, creating clear edges that tell the flowing resin where to stop.
Here is a Doming How-To Video:
If you're doming larger pieces (over 2"), especially with the Clear Photo Film, or long pieces, you should stabilize them by adding a stiff paper like cardstock to the back before adding resin. This will keep your pieces from curling up as they set. You can use a few small pieces of double-stick tape to attach them. Here's an example: