The 7-8-9 Method requires a little practice initially, but once you have established your distances you will be able to rely on it in the future.
Imagine as you address the ball that you have a large clock in front of you. Learn to swing your left arm (right arm for left handers) to the various "hours" of the clock. The 7 o'clock position is pictured above.
Notice in the photo above that there is a slight wrist cock. This is important as you need to cock the wrist to help deliver a slightly downward blow through the shot. Practice hitting shots like this until you can consistently hit shots a certain distance. This will become your 7 o'clock shot.