This is how I added magnetic snaps to the flap for Lily's Diaper Bag. You can use this tutorial for adding magnetic snaps to almost anything.
Fabric for flap: 2- 7" squares
Fusible Fleece/Heavyweight Interfacing: 1 - 6.5" square
1 magnetic snap
Place 7" squares together, right sides facing
Sew along 3 sides
Clip corners, turn
This is what it should look like: