To get a USGA Handicap Index™ you need a compliant Golf Handicap Tracker to calculate it for you and to provide access to the required USGA® Clubs and Associations.
The Handicap is issued by a Golf Club, so you can't get one by simply calculating it. The idea behind it is that there needs to be someone (real person) overseeing each golfer's golf activities
The Golf Handicap is intended to reflect the potential ability of a player. Meaning: "On a good day, what score would this player shoot". Most golfers incorrectly believe that the handicap is supposed to be a reflection of the average score of a player, but it is not.
Your Handicap Index® is calculated by averaging the best 10 rounds out of your last 20. Those best rounds are determined by calculating what is called a Handicap differential.
Handicap Differential = ("Score" - "Course Rating™") x 113 / "Slope Rating™"
Once you average your best 10 scores, then you multiply the result by 96% to get the final result.
EXAMPLE
In the example, you see the the last 20 scores of a player and the respective differentials in the bottom.
The best 10 Hdcp diff. are marked in green.
Handicap Index® = 8.7