For a heart shaped egg I start by gently pressing the egg with my thumb into the corner of one piece. The egg will flatten a bit.
Next, place the second piece, corner down, on to the egg and press gently to make an impression. Be gentle, too much pressure will break the egg.
I held it together with rubber bands. Do not double the rubber bands, it will be too much pressure.
Place the egg in the refrigerator for about 30 min and let the egg harden.