if you use a wall, say far away from it. as if you were behind the baseline and it was just beyond the service-line on your opponat's side of the net.
staying too close will make the ball come back too quickly (as if an opponant is vollying you) and you will be forced to compromise your form.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM