STEP 1: Put on the shirt and mark the end of your shoulder.
STEP 2: Take shirt off. Draw a curve from the shoulder mark to the armpit, under the original armhole seams. This will be your new armhole. Fold the shirt in half and cut your curve, cutting off both sleeves together.
STEP 3: Turn shirt and both sleeves inside out. Arrange so that the right sleeve is with the right side of the shirt, and the left sleeve with the left side of the shirt. The buttonholes on the cuff should be on the bottom.
STEP 4: Pin the sleeves back onto the shirt. Do this by matching the yoke seams together at the top of the shirt with the yoke seams on the sleeves. The right sides (the outside of the shirt) need to face each other.
You'll have a hole at the armpit because the hole of the sleeve is smaller than the armhole of the shirt. Just try to pin together the sleeve and the shirt as much as possible to keep this hole small.
STEP 5: Sew the sleeves and the shirt together
Go slowly since you're sewing a circle.
STEP 6: Put shirt on inside out. Draw an angled line from just underneath a boob towards the side of the shirt. This will be the dart.
If you notice the dart extends onto a shirt pocket, either start the dart below the pocket, or remove the pocket first.
STEP 7: Take the shirt off. Use a ruler to extend the line to the edge of the shirt.
STEP 8: We need to copy the dart onto the other side of the shirt. If you used chalk, just fold the shirt in half and transfer the chalk line to the other side. If you want it accurate, take these three measurements
a) The horizontal width from the top of the dart to the side of the shirt.
b) The length down the side of the shirt from the armhole to the horizontal line I drew with step A.
c) The length down the side of the shirt from the armhole to the end of the dart.
As you can see, I got a) 17cm b) 5cm c) 23cm
Use these measurements to draw an identical dart on the other side of your shirt.
STEP 9: Pin the darts.
STEP 10: Sew the darts just under the line you've ruled. How much under is up to you. As a general rule, if your boobs are smaller, sew closer to the line. If your boobs are bigger, sew further from the line. Curve out at the top of the dart to close it.
STEP 11: Turn the shirt right side out and try it on. If you're happy with your darts, turn the shirt inside out again and go ahead to cut off the excess material above the dart line. Iron open the dart seams.
STEP 12: Put the shirt on inside out. Mark where your waist is and then mark where the underside of your arm is. Be a little generous as you can always keep narrowing the shirt if it's too big.
STEP 13: Take shirt off. Draw a straight, angled line on your sleeve from the armpit to the cuff. Then draw a curve from the armpit to the bottom of the shirt. The widest part of the curve is the mark your made of your waist. Cut.
STEP 14: Pin, matching the underarm seams and the cuffs of the sleeves. Pin the trunk from the armpit down. Because of the dart, there will a bit of the back shirt hanging longer than the front.
STEP 15: Sew. Try to keep the armhole seams and dart seams open.
STEP 16: Iron open all your seams, try it on!