At contact, your arm should become as straight as possible. (Sometimes this
is not possible due to the height of the shot). The racquet should
be parallel to the net. Try to produce a sort of inside out slice (very
similar to a backhand slice). Your side should be leading into the shot
as much as time has allowed. The swing will tend to come across
and downward in front of your body. Your wrist will remain locked back so
as to return the ball upwards as much as possible.
The follow-through will show your racquet finishing across the ball, the
racquet face remaining open (or upward), your wrist remaining firm. It is
common and expected to see a side spin on the ball. In fact, after the ball
bounces it should spin a little to the outside.
With practice this volley can be extremely safe--and will be very natural.
But if you try hitting a forehand volley when the ball is hit into your
body, you'll be saying, "OOPS, I'm so sorry!"