Whether you call it a snuggie, slanket, cuddle wrap or blanket with sleeves, it's all the same: a big fleece blanket with sleeves. They are popular because you can stay warm and use them while reading, on the computer, drinking tea or watching TV. If you would rather make your own, you can purchase fleece rather inexpensively at fabric stores.
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Get a blanket, fleece fabric or two blankets, depending on the size of the person. Measure the person who will use the blanket from her shoulder to the ground. For example, let's say that's 4 feet, or 48 inches. Also measure her arm span -- fingertip to fingertip with arms outstretched. Let's say that's 4 feet 6 inches, or 54 inches.
Fold your fabric or blanket in half with the right sides together. Your fabric should be longer than it is wide when it is folded in half.
Use a fabric marker to draw a thick "T" shape on the fabric. It should look like a T-shirt only longer. The arms of the T will form the sleeves, and the middle of the T will go around the body. Be generous with the sizing. You don't want a form-fitting blanket.
Seam along the lines of the T, leaving the bottom and the ends of the sleeves open. Cut a hole in the top for the head to go through.