These are super simple, and will vary in price, mine were free.
Use the pliers to pull out the edges of the cap, and super glue the magnet inside.
If the magnet you choose is thick enough, you don't even need to ply out the edges. I had bought mine for another project, and I liked the look of the edges. Just make sure your magnet sticks out farther than your cap edges, you don't want to scratch your fridge surface.