Holding one end of the 20g wire against the mandrel, wrap it 4 times, leaving the other end long.
Pro Tip: I use a permanent marker as a mandrel, since I’ve already determined it’s the diameter I need. If you decide to use a mandrel, make sure you measure a smaller size than you need, because when removing the wires from the mandrel, they tend to open a bit, enlarging the ring about 1-2mm. The ring in this tutorial is about 19mm inner diameter and the permanent marker measures 17-18 mm, just to give you an idea.
If there are any more questions about the size, do please feel free to ask, and I’ll be more than glad to answer and help, if necessary.