Creating a great Army costume can be as easy as a visit to an Army surplus store. There you should be able to find everything you need, from camouflage clothing to rank insignia and ACU patches to hats of all kinds. If you don't have one near you, get the clothing from any department or thrift store, and order hats and patches online. Depending on how much research and how much you want to spend, you can make an authentic costume or something easy and fun for Halloween.
Sew U.S. Army patch over left breast pocket of jacket.
Sew rank insignia to sleeve of jacket.
Sew an ACU patch to the front of a hat.
Sew any other patches, pins or medals you want.
Get dressed in your T-shirt, trousers, and boots. Tuck in the shirt, and put on the belt. Tuck trousers legs into top of boots.
Remove buttons from blazer. Replace with gold buttons. Add a gold button to each pocket flap.
Sew a strip of gold braid around each sleeve, about three inches from the edge.
Sew rank insignia to sleeve. Sew U.S. Army patch to front of blazer.
Sew gold braid down side of trousers.
Pin U.S. Army pin to front of cap.
Add any other patches, pins or medals you want.
If you can't find a blazer with four pockets, or if your pockets don't have flaps, you can make fake ones by hemming pieces of pocket or flap shaped cloth in a matching colour and sewing them to the front of the blazer.