Cut two pieces of iron-on fabric to the size of the magnetic buttons. The iron-on fabric will make the seam more stable.
Place the pieces of iron-on fabric on the back of the inner and outer fabrics on the spot of your choice and fix them with a hot iron.
Use the plastic square to sew the magnetic buttons onto the iron-on fabric.
Overlap the two fabrics top side on top side and pin the piping to them (cord towards the inside).
Sew along the piping and the edge of the fabric while leaving out the last part in order to still be able turn the pouch inside out.
Turn the fabric inside out, fold it and fix it with pins in order to create the pouch.
Sew the edges and the flap of the pouch.