Well, I am not sure what the official wayis, but the way I
was taught was to take a 48 inch steel ruler. You can buy
these from Home Depot or one of the golf supply
houses.
Put your club in its playing position, put the ruler flat against the back
of the shaft so that the bottom of the ruler is resting on the floor
touching the heel of your club. Measure at the top of the grip
cap.
There are other ways of course, There is a True Measure device,
and Mitchell sells a nice jig to put your club in and then measure.
But the above way should be close enough.
Hope this helps
Garry