Taken from Wikipedia:
Green in regulation (GIR) A green is considered hit "in regulation" if any part of the ball is touching the putting surface while the number of strokes taken is at least two fewer than par (i.e., by the first stroke on a par 3, the second stroke on a par 4, or the third stroke on a par 5).
Do that on every hole on an official course. Obviously, this will be easier if you play a Custom game, turn off the wind, play from the closest tees, and play on Easy.
What else should I add? I've only been playing the Tour and have come close a few times. I haven't tried to do a custom game yet because I'm hoping to get it on Tour.