Knit fabric stretches, especially when it is cut. The trick to turning a crew-neck T-shirt into an off-the-shoulder over shirt is to cut as little as possible. Cut the neck out of a short-sleeved shirt and wear it over a neon-colored tank top so that just one strap shows. Or use a long T-shirt to make an off-the-shoulder swimsuit cover up. Cut the neck out of a sweatshirt and wear it over your leotard for a dance warm up. The open neck keeps you from getting too hot.
Cut the collar off a T-shirt or sweatshirt. Cut just below the stitching line.
Try the shirt on. Pull the shirt down over your shoulder to gently stretch the neckline. If the neck opening is large enough, you are done.
Cut the neck 1/2 inch below the previous cutting line if the opening was too small.
Continue to try on the shirt and cut the neckline 1/2-inch until you are pleased with the shirt.
The opening in the shirt will stretch as you wear it. Stop cutting when the neck is still a bit higher than you want it to be.