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Tuck the sleeves inside the shirt first, so that you have nice, curved edges for the arm holes. Place the shirt on top of the craft foam. Trace around the shirt, including the arm holes, stopping a few inches below each arm pit. This will be the base for your chest plate.  If you can't find craft foam, thin, flexible cardboard may also work. Make sure it's white. Stay on the safe side, and add a few extra inches to the top of the shoulders. Test it over your own chest. The top edges should wrap over your own shoulders. The bottom edge should extend a few inches past your armpit. Don't worry about it wrapping all the way around your torso. Make sure that the breast cups are touching each other in the middle (this forms the cleavage). The top edges of the breast cups should be just below the arm pit of the chest plate. The bottom edges will likely hang over the bottom edge of the chest plate. The thinner part of the breasts should be pointing upwards. The thicker, fuller part should be pointing downwards. Once the glue dries, flip the chest plate over. Trim off the excess chest plate until you can see the entire inside of each breast cup again. This will make the breasts more comfortable to wear, especially if you are a girl.

Summary:
Trace the upper part of a t-shirt onto a sheet of white craft foam. Cut the chest plate out. Glue the breast cups to the bottom of your chest plate. Trim off the excess chest plate behind the breast cups.