Fill your iron reservoir with water, and plug in the iron. Some irons do not have a reservoir, but if yours does it will convert the water to steam or spray a stream of water when you press a button. This will take the place of a spray bottle.
Spread the T-shirt on the iron board, placing the narrow side through the neck of the T-shirt.
Spray the shirt with water, either from your iron reservoir or from a spray bottle. This will loosen up the wrinkles and make it easier to press.
Apply the iron to the shirt using long, slow strokes. Rotate the shirt around the ironing board as you iron until you have pressed the entire torso section of the shirt.
Remove the shirt from the ironing board and put the narrow end of the ironing board through one of the sleeves. Use the same slow strokes you used on the torso section as you rotate the sleeve around the ironing board. Repeat with the other sleeve.