Usually costs about $100 but they'll apply some or all of that to new clubs if you buy through the fitter.
The fitter should measure your hand, fingertip to ground, height... He'll ask about your current swing and what you want to improve. He'll ask about injuries or anything inhibiting your swing.
Hit balls with your own club (probably 6 iron if you're fitting irons) with some type of launch monitor that analyzes the ball path. Then he'll probably have you hit 5-10 shots with various clubs based on how you're hitting the ball.
You'll get feedback (spin, launch angle, carry distance, total distance, etc) with each of the clubs you tried.
That's about the gist of it.
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