Ask someone to help you or try to do it by yourself if you are still flexible enough to measure.
Place a soft measuring tape at the outside edge of your shoulder in the middle (where your shirt seam is). Pull the tape measure tight and come down across your back and around to your opposite hip bone. Make sure they come to the top of your opposite hip bone in the back but close to the side of your body as shown. The side to measure to is where the outside seam of your pant leg is. Then look to see where you fall on the sizing chart above. This size of sling you order should be similar to your fitted t-shirt size when not pregnant.