Step 4Sew the cut-up onto the shirt-that-fits
iImage Information

Take the cardboard or whatever out of the shirt and put the shirt into the sewing machine. I like to use contrasting thread and here I've used white to match the shirt-that-fits. Use a fairly long stitch length. If the shirt fabric is very stretchy, or if the shirt is tight when it's on you, use a slight zig-zag stitch so the stitching line has some stretch to it. The stitching line will necessarily not stretch as much as the fabric but for most cases this is find. If the shirt is VERY tight on you, this probably isn't a good technique to use to alter it.
Check out the comments on the pictures for ideas about avoiding some common problems, like sewing over pins or having the lines come out uneven from one edge to the other.