This is one of the things that coaches loooove to say “follow through over your shoulder. that’s good. doing great.”
In my opinion, there is no one way to follow through “properly”. Nobody in the pros thinks about their follow-through. The follow through is an after-effect caused by the mechanics of the stroke. After you make impact with the ball, and it has left the strings of your racquet, the subsequent trajectory of your racquet – whether it goes sideways, up, or down – is inconsequencial.
Sometimes you don’t want to follow through over your shoulder – sometimes you follow through with a downward trajectory, or even over your head! Sometimes if you’re running out wide, you have to lassoo the racquet around your head because you have to hit the ball with high looping spin.
I’ve seen some coaches recommend following through low to the hip in a “wind shield wiper” motion.
This is neither good nor bad. To me it indicates poor extension through the shot – maybe that’s what you need if you are very close to the net and are trying to hit a spinny sharp-angle shot crosscourt. But it’s not what you want if you are rallying from the baseline and trying to hit a deep looping ball. It depends entirely on the situation.
GENERALLY, when you’re hitting from the baseline, and hitting with top-spin, the racquet will travel from below the ball to above the ball, so the racquet will be traveling on an upward trajectory by necessity. When this upward motion is combined with your forward movement and body rotation, the natural path of the racquet is to finish over your shoulder. But to actively think about your follow-through is missing the point. Think of your technique, and use your follow-through tendencies as indicators for your swing mechanics prior to ball contact.