Fill a plastic spray bottle halfway with vinegar. Fill the bottle the rest of the way with cool tap water.
Unbutton any buttons that hold the collar of the shirt together. Unfold the collar and place the shirt on the ironing board so the front faces down.
Moisten the collar with your vinegar and water solution. Place a damp cloth over the collar and warm your iron. Refer to the care instructions on your shirt's tag to determine how hot to heat the iron.
Iron over the damp cloth using long, smooth passes. Remove the cloth to see if the crease has been smoothed. If not, repeat steps three and four until the collar lays flat.
Fold the collar back over and hang the shirt or wear it immediately to avoid further creasing.