Cool cake for 10 minutes in the cake pan, then remove it to a wire rack to finish cooling. Cool the cake in the refrigerator for at least two hours or preferably overnight before decorating. I like to bake cakes one day, then decorate them the next. This helps release the steam in the cake and the frosting holds on the cake when chilled, otherwise, it melts.
Place a dollup of frosting on a small piece of cardboard or a tray, and set the cake on top. This keeps the cake from sliding around while decorating.
Frost the cake using a small offset spatula. Smooth out the frosting as best you can all over the top and sides of the cake.
Using a knife, draw a line about 5 inches long and 4 inches tall for the blue part of the flag in the upper left corner. Place blueberries in that box, pick the best larger ones and place them with the smooth side up.
Starting at the top of the cake, use large raspberries pointed up and make long rows of red, leaving 1" white stripe in between each row of raspberries.
Fill a large piping bag with a large 1M star tip and frosting. PRO TIP: rest the piping bag in a tall sturdy jar or glass and pull the piping bag down, it really helps to fill it up. Pipe stars on blueberries for the stars of the flag working in slightly offset rows like the flag.
Pipe stars in between rows of raspberries, and decorate the sides of the cake if you wish. You will have some frosting leftover, it freezes great in a zip-top bag, and can be used for another dessert.
The cake will hold well overnight in the refrigerator for 1 day, and possibly 2 days if your berries are really fresh. I would decorate the cake either the day of your party or the night before. I have also had guests decorate it during the party which was SO much fun!