Darts are an alteration technique used to help make boxy shaped shirts more fitted. Though darts can be employed in the front and back of a top, fullness is commonly taken from the back of the shirt, as back darts allow a dress shirt to fall smooth and flat in the front. Darts can be an effective bulk-reducing alteration --- a quick pinch, pin and sew can help the shirt hug the body's form and provide a more tailored look.
Try the dress shirt on to measure how much fullness should be taken out of the back. Pinch the excess fabric in two places on either side of the shirt, close to the waist area. These two pinches should be approximately 2 inches from the side seams. Because it might be difficult to take this back measurement by one's self, employing the aid of a helper might produce a more accurate alteration. Pin the pinched fabric, as this will be the width of the darts.
Take of the shirt and lay it on a flat surface. Mark either side of each pinch with a piece of dressmaker chalk, and then remove the pins. These dashes will outline the dart's width. With a ruler, measure the point that is directly in between these two width marks, and dot the centre of the dart. Then draw a vertical line at this centre mark, 6 inches in length, 3 inches above and 3 inches below the waistline. To finish drafting the dart, draw a line from dart's width marks to the top and bottom of the 6-inch line, forming a diamond shape.
Fold the dart along the vertical 6-inch line and pin one side of the diamond shape to the other. This folded diamond shape will be the sewing line of the dart. The dart's shape ensures that the fullness is cinched at the waist, and then gradually tapers outward into the body of the shirt.
Check that the darts and altered fit are correct by trying on the shirt. The shirt should be fitting more snugly at this point and the darts should be falling vertically. Inspect the back of the shirt to make sure there isn't any abnormal pulling or puckering from the darts. Also, the front of the shirt should be falling smoothly.
Sew the two darts along the seam line using the pins as the guide. Start at the bottom of the dart, removing the pins one by one until reaching the top. Once finished, iron the darts inward towards the centre of the dress shirt. This will help smooth out the shirt's newly tailored lines, finishing the alteration.