Step 2: Fitting in
Place pins down the side seams, about 1/2 inch on the outside edges of the sample shirt (for seam allowance). Stop pinning at the armpits and make sure you mark that point (marked in the photo below using a blue pin, though you can also use chalk to mark this spot). Next, mark the point on the shoulder seam where the sample shirt's sleeve and body are connected (white pin on the right in photo below). Lift off the sample shirt. Following the original curve of the sleeve that you cut off, place a pin to connect the armpit to the shoulder (pink pin in photo below).
You can do this on both sides as shown below, or you can save time by pinning only one side and then folding the shirt in half lengthwise before you start cutting.
Cut along the pinned lines.