Fashion-conscious individuals tend to pay extra attention to details others might overlook, like buttons, accessories and collars. A stiff collar can improve the look of a shirt and even make it appear newer. Sweatshirts, usually made out of soft cotton, can also have their collars stiffened, as with an ordinary dress shirt.
Lay the sweatshirt flat on the ironing board.
Repeat ironing the collar several times. Use a hot iron.
Apply a bit of starch spray to the collar.
Run the iron over the collar again.
Flip the sweatshirt over and apply the same process to the back of the collar.