Determine what shaft flex description best describes you. For example, if you are a woman or a senior, you'll likely narrow your shaft flex to woman or senior flex. If you are an avid golfer, you'll want an extra stiff or stiff shaft. If you are a weekend golfer, you'll likely choose a regular flex.
Hit your driver. Go to a driving range and hit 10 to 15 driver shots. Write down the distance and calculate an average. If you consistently hit 250 yards or more, stiff shaft flex is best. For shots 230 to 250 yards, a regular flex is appropriate. For shots 200 to 230 yards, choose a senior flex. Finally, if you hit less than 200 yards, choose ladies.
Characterize your swing. If you can't self-assess your swing, have a friend familiar with the game of golf analyze your swing. If your swing is smooth, you'll need a softer flex. If your swing is hard and explosive, you'll need a stiffer flex.
Watch which direction your drives go. If you slice your drives right, you'll want to choose a softer flex. If your drives go left, a stiffer flex is appropriate for you.
Calculate your club head speed if possible. You'll have to visit a golf store that has a swing analyzer. Choose extra stiff for club head speeds of 93 mph or higher. Select stiff for a club head speed of 84-93 mph. Choose regular for a speed of 75-84 mph. Select senior for a speed of 60-75 mph. Finally, choose ladies for ad club head speed of 60 mph or less.