If you have a ring that fits you well, drop it onto your ring the mandrel and see what size it is. If you're not sure what size you need, use a piece of string to measure the circumference of your finger and then put the string around the ring mandrel.
Plan to make your ring a little bigger than your actual ring size to accommodate the wire wraps.
Cut 18 inches of 20 gauge, round, dead soft jewelry wire. You can use any metal, including colored craft wires. I prefer silver filled, bronze or copper wire.
Find the approximate center of your wire. Set it against your ring mandrel a little bigger than the right size marking and hold it in place with your thumb.