If you're wearing your sweatshirt during athletic activity, having holes for your thumbs in the sleeves ensures that the sleeve will keep your arm and wrist covered, and that you won't have to deal with the cuff getting in your way. There's plenty of athletic clothing on the market that has pre-made thumb holes, but you don't need to go out and buy any. Your sweatshirt will do fine. Making a thumb hole in the sweatshirt is a simple task that you'll be glad you took the time to do, especially when it comes time for those winter morning jogs.
Put your sweatshirt on. Make sure the sleeve is all the way down your arm, and not at all bunched up.
Grasp your chalk and make a mark approximately halfway between the beginning and the hem of the sweatshirt's cuff. Repeat on the other sleeve.
Take off the sweatshirt and use the point of your scissors to bore a hole in the center of your chalk mark. Do this by opening the scissors and using the point of one blade to pierce the fabric. Twist the scissors in a circular motion as you break through the fabric, to make the hole. Make sure you're only piercing the fabric with the chalk mark, and not going through the bottom side of the cuff too.
Put one of your thumbs in the hole to further stretch it out. Sweatshirt fabric is very stretchy and over time, repeated use will create the perfect size hole for your thumb. Because sweatshirt fabric doesn't fray, you don't have to worry about sewing the edges and can leave them rough as is. Repeat Steps 4 and 5 with the other sleeve.