Measure and cut fabric as follows: BLUE cut one piece at 5x4.5 inches. RED cut two pieces at 6x1.5 inches and one piece at 11x1.5 inches. WHITE cut one piece at 6x1.5 inches and one piece at 11x1.5 inches.
Lay the pieces out on the sweatshirt, with the edges of the piece touching to create a flag design. Pin the pieces onto the sweatshirt using a lot of pins. Be sure to pin through the front of the sweatshirt only.
Begin sewing on the bottom stripe. Sew all the way around it, 1/8th of an inch from the edge. Create sharp corners by leaving the needle down, lifting the presser foot, and rotating the fabric. Backstitch at the beginning and end.
Remove the sweatshirt from the sewing machine and trim threads. Then reposition and sew on the next stripe. Continue until you have sewn on each piece.
Take care when sewing that you are only sewing through the front layer of the sweatshirt. Also take care to smooth out the fabric ahead of you as you sew.
Wash the sweatshirt in a washing machine. Trim any long threads. Then dry in the dryer, and trim threads again. The raw edges of each fabric piece will fray, giving it a worn look.
You may need to trim a few threads after each wash for a bit. Wash inside out from now on to prevent fraying past the machine stitching.