If you are having troubles cutting straight, use a ruler or measuring tape as a guideline.
Try to use a thread color that closely matches your t-shirt. If your sewing machine has a knit fabric setting, try to use that; it usually looks like a standard straight stitch that’s broken up by V shapes. Remove the pins when you are done. Also, remember to snip off the loose ends of the threads.
Be careful not to lose the other end of the elastic inside the shirt hem.
Once you have the elastic back through the gap, you can sew the gap shut for a nicer finish.
Try to use a stitch intended for sewing stretchy fabrics. It usually looks like a straight stitch, but is broken up by V-shapes.
At this point, you can sew the gap shut for a neater finish.
