Standard cake mixes will not hold up as well for this technique. Look for a heavier, more dense cake mix that has pudding in the mix or a pound cake.
Follow the directions in this video of an online class.
Spray loaf pan with non-stick spray.
Pour 1/2 in worth of batter in pan for main cake.
For cut-outs, use a thin sheet pan and fill with 3/4 in worth of colored batter in pink. Bake.