For flag: Position cake on foil-wrapped board. Ice smooth. Mark 3 1/2 x 4 3/4 in. wide corner area; cover with blue gel. In corner, mark 11 vertical rows, about 3/8 in. apart; pipe tip 3 icing dots, alternating rows of 5 and 4 dots. Flatten slightly with spatula dipped in cornstarch. Mark 13 stripes, 1 in. wide; outline and pipe in alternating stripes with red gel. Add tip 21 star bottom border.