Bake the cake following package directions. When the cake is cool, level the tops and spread jelly on top of 1 layer. Place the second layer on top of it. Cut the cake into quarters. You will have four 2-layer squares. Frost each square completely. With decorating icing outline all 12 edges of each square. With decorating gel write A, B, C, and D on the front of the blocks and write your message on the top.
When the cake is cool, cut a 3-inch-wide wedge from the cake and remove it. Place the 2 cup cakes below the round layer. Frost the entire shape. With decorating gel outline the carriage shape and add radiating lines for the carriage hood. Draw the wheels and spokes.
Write the baby's name and birth date on the body of the blocks.
Also, you can't have a great edible cake without a great matching Lil' Baby Cake for a baby shower diaper cake.