You can make them any shape; these will be circles. I trace the shape onto heavy cardboard and thin batting. For these I used a crazy quilt front, but you can use cross stitch or hardanger or any other needlework or even plain fabric. I trace the shape onto the base fabric and follow the line with a running stitch, so I can see how far to embellish.
After completing the stitching, I use heavy thread to do a running stitch a little larger than the circle, then pull it tight to cover the cardboard and batting.
(Same thing for the front and back).
I use a little piece of ribbon for the hanger and attach it before I put the two sides together.
Using a ladder stitch, I put the front and back together. When the thread is pulled tight, the stitches are barely visible.
The pins slide in between the two sides, making it perfect for carrying around without getting stuck by the pins.
Here are the three I made. If you give it a try, leave a comment with a link to your pin keep so others can see.