Draw a square on the wall with pencil and frame it with masking tape.
Then apply 3 to 4 coats of magnetic paint within the frame (adhere to the instructions on the can).
Since I painted on ingrain wallpaper and was afraid the magnets wouldn't adhere well later (which was totally unfounded), I decided to use a paint roller.
Unfortunately I left some stains on the wallpaper where I didn't want them.
But I was able to remove them easily with a wet cotton cloth!
Thereafter, you can just paint over it with a regular wall paint.
You might need two coats to fully cover the grey magnetic paint.
Decorative frame around the magnetic paint
Remove the masking tape and trace the outline with a small brush and chalk paint to hide the jagged edges around the magnetic area.