Here is how I did it. You will need clear glass half marbles. I don't know if that is the correct name for these or not, but for today they are half marbles.
You need some decoupage glue or other glue that dries clear. I used Mod Podge matte finish. You can also have a gloss finish glue too to give them extra shine.
I didn't really know what was the optimal thing to use on the back-paper or fabric and what kind. So I just experimented. Thin tissue paper or decoupage paper would probably work very well, but I didn't have any of that. So I used decorative paper and cotton fabric.
And then you will need magnets. Whatever size you have is ok so long as they can't be seen from the front.
Make sure your marble is clean and doesn't have bubbles or scratches in it. Put a thin even layer of glue on the flat side of your marble. Lay in on your paper or fabric, making sure your design is centered. Let dry.
Once they are thoroughly dry trim off the extra paper or fabric. Then use more glue on the edges. When this is dry you can add your gloss finish if you used thin paper.
Glue your magnet on the back and you're done! Here are some I did using fabric. And then I glued them on a felt flower and then glued the magnet on the back.
Well the pictures didn't show them off too well. You'll have to trust me that they are cute. I hope you try this.