To make these cakes, visit the cakeculator, select your flavor of cake, and select "Cake: half-sheet pan (18x13), 1 layer". This will yield a single half sheet pan of cake, which you can then use a cookie cutter to cut shapes.
For the frosting, use any type of filling that you like. Buttercreams will work best if you're a beginner as they are stable and easy to pipe. Select the flavor from the dropdown menu.
Check out the video tutorial here:
... the nerdy baker behind the videos and recipes here. I coded this site to not only share my recipes with you but also to build some helpful tools for bakers.