The average player finds swinging overhead an unnatural stroke.
Usually the muscles are not very strong, particularly on the backhand overhead volley.

1. Volley side to side

Standing at the T move from forehand to backhand swinging overhead.

2. Volley figure of 8

Figure of 8 swing at hip height, then gradually move the swing to shoulder height, then overhead height.

Video clip Figure of 8 Volley

3. Return of Serve swing

Standing near the half court line move as if to overhead volley a return of serve.

These exercises are designed to compliment training sessions, and if attempted for a short time each day at home will improve your  game a lot.  At first you will be uncoordinated, and awkward.  Try to emulate your game, with particular emphasis on footwork, racket work, and fluent movement.

Overhead shots are not usually struck one after the other, and mostly  followed by or preceded by another ground shot.  This section will mostly concentrate on efficient movement and racket work,  moving into various positions within the court, as nominated, to strike the ball, eg:

  1.    Forehand drive, backhand volley

2. Backhand drive, forehand volley.

3. Backhand lob, forehand volley.

4. Forehand lob, back hand volley. 
  
A good practice session would be these drills followed by solo drills, pair routines or restricted games.