We did these "stained glass" circles at the family reunion.
Here is the process:
I folded a piece of Contact Paper in half and traced a plate on top. Cut through both layers for two matching circles. A little tape or paper clips can be used to help hold the edges together for easier cutting.
Peel one backing off and tape it sticky side up on a table. Just tape a couple of spots along the edge to hold it flat. I used masking tape, which worked well.
After collecting nature bits (try to find things that are fairly flat), arrange them in the middle of the circle in any design. Leave a bit of a border around the edge so they will seal well.
When your arrangement is done, peel the backing off of the other Contact Paper circle and position it on top of the first circle, pressing and rubbing to seal them together. Pull off the tape that taped the bottom circle to the table. Seal those spots. Trim edges if necessary.
Punch a hole in the top. Add a string to hang. This could also be done with magazine cut outs, cut or torn tissue paper, family or trip pictures, etc.