If you wish to hang your painting, it's best to attach a clip to the back before you start decorating (you could damage the relief by attaching a clip once the painting is finished).
Measure and draw a rectangle inside the support to delimit the "magnetic" part.
Paint this rectangle in 2 coats with the magnetic paint.
With the relief paste, and using a simple flat brush, draw clouds in relief in the upper part of the picture, and a relief in the lower part, accentuated on the sides.