The players line up behind the deuce side baseline, having a tennis ball in their hands; the coach is inside the service line, ad court (see graphic below).
At the coach’s command, the first player in line will hit a serve then move up towards the service line where she’ll receive a short ball fed by the coach, then keeps going up towards the net and volley another ball fed by the coach.
She’ll then exit the court, get a new ball and move to the end of the line making room for the next player to follow.
Repeat the drill on the ad side too.