This is a post of tips for altering my dress shirts for the next time that I do it.
Measurements and Pattern Drawing
Take the waist and hip measurements from an existing shirt. Take the chest, biceps, and sleeve length measurements from the body since those are too variable based on the shoulder dimensions.
Avoid flaring at the hips too much.
Try to avoid sleeve alterations as much as possible. They are a pain to do.
When drawing the curve from the chest (arm hole base) to the waist, don’t draw a smooth curve. Instead, sharply curve inward from the arm hole down about a third of the way, and then go more or less straight to the waistline mark. This is because the waistline circumference is about the body circumference all the way up to the base of the chest.
When measuring how much to take in, turn the shirt inside-out and draw lines from the stitching line that will not be opened up (see below). Most of the time, this will mean drawing the altered lines on the inside back of the shirt, measuring from the farther seam on the sides.
Preparing for Sewing
Unstitch only the stitching line that opens up the flat-felled seam from the inside. (That is, turn the shirt inside-out and unstitch only the line closer to the fold.) This will save the trouble of having to re-align the fabric pieces when resewing. This is the case regardless of the method of flat-felling the seam.
Also unstitch the bottom hem.
Sewing the First Line
Sewing the first line is pretty obvious. For shirts where both sewing lines on each side are chain stitches, it may be necessary to open up a small amount of both sewing lines at the point where the new line will intersect the old one.
Ideally, the first new stitching line should go up to and abut the existing stitching line.
Cutting the Fabric
Try the shirt on before cutting! And keep in mind that flat-felling the seams will make the shirt a little tighter.
Remember to consider which edge is being folded over before cutting.
At the point of cutting, the original seam that was not removed can now be removed. Usually it has to be taken out, because the new seam allowance might go over the existing seam.
At the arm hole seam, cut the edge to be folded over generously, to account for the difficulty of folding over all the extra fabric. This is particularly true for thicker materials.
Sewing the Second Line
There’s not really much advice to this line of sewing.