You can purchase a tombstone shaped cake pan at most cake decorating
supply or large craft stores.
Mix and bake cake according to directions on cake mix box. Let cool.
Remove from cake pan and lay on a flat cake board. A little trick is
to cover the board with aluminum foil or parchment paper and you can
take the slices off easier.
Decorate according to direction that came with the cake pan. Using
disposable pastry bags for frosting makes clean-up easier. Using a
frosting spatula available at craft and cake decorating stores makes
applying frosting easier, too.
You can use canned frosting or make a simple frosting like this one:
1 cup of shortening
2 cups of powdered (confectioners) sugar
1 teaspoon meringue powder
1 tablespoon cold water
Using a hand mixer, blend
the ingredients together until silky smooth. Separate into bowls and
use paste food color to make the needed frosting colors.