Step 1: Preheat the oven as directed by the instructions on the cake mix (usually 350F).
Step 2: Mix up both boxes of cake mix, pouring the cake batter from one mix into the football (or oval) shaped pan, and the batter from the second box of mix into the 9 x 13 inch rectangular pan. Slide the pans into the oven and bake as directed by the instructions on the cake mix box.
Step 3: While the cakes are baking, wrap the cake board with decorative foil.
Step 4: Divide the white decorator icing and mix 1 ½ cups of the icing with green food coloring to turn it green. Reserve the remaining ½ cup of white icing (you wil use this white icing later to make the details on the football cake).
Step 5: Once the cakes are baked, turn them out of the pan let them cool for a a few minutes. Level the tops (the side that was up they were baking) of each cake with a serrated knife. Leave the cakes to cook completely.
Step 6: Put a small amount of icing to the middle of the cake board; this will act as glue to keep the cake in place. Place the rectangle cake in the center of the board, cut side face down, on top of the icing.
Step 7: Ice the rectangle cake with the green icing.
Step 8: Place the football cake in the center toward the top of the rectangle cake, cut side down.
Step 9: Ice the football cake with the chocolate icing.
Step 10: Let the cake sit for a few minutes so the icing can dry. Once dry, use a paper towel to smooth the icing on the football.
Step 11: Add some of the green icing to a pastry bag with a Wilton #233 tip (or similar) on the end. This is a round tip with several small holes in the end to let the icing come out in thin ribbons to look like grass. Use the tip to make blades of grass over the top of the rectangle cake with the green icing. Squeeze the pastry bag, lift and release to make the grass. Repeat until the exposed part of the rectangular cake is completely covered in grass.
Step 12: Add the white icing to a pastry bag. Attach a round piping tip (like Wilton #5) to make the stitches to the football. Pipe one long string down the center top, and several shorter stitches that cross through the long stitch, as pictured below:
Step 13: Use a star tip like Wilton #16 to add a double row of stars on each end of the football to make the stripes.