Measure the desired ring size using a ring sizer on the finger you wish to place the ring.
If the mandrel is not calibrated and does not indicate ring sizes, place the appropriate ring of the ring sizer on the mandrel. Once the ring will no longer move down the mandrel, use a pencil to make a line at the bottom of the ring shank.
Place the handle of the mandrel vertically in a vice. Tighten the vice until the mandrel will not move when struck.
Slide the ring to be enlarged down the mandrel until it can't go any further.
Tap all the way around the ring lightly with a rawhide mallet until the ring stretches halfway to the ring size marking on the mandrel.
Remove the ring from the mandrel, turn it over, then place the ring back on the mandrel until it can't go any further. Tap all the way around the ring lightly with the rawhide mallet until the bottom of the ring shank reaches the desired ring size mark.
Remove the ring from the mandrel and try it on. Place the ring back on the mandrel and tap it lightly two to three times if minor adjustments need to be made.