So you're in a baseline rally and your opponent has made a mistake. She's hit you a short ball that you are able to attack.
If it's really short and sitting up nice and high, go ahead and try for a winner. Don't overhit the ball - always swing within yourself - but hit the ball firmly into a part of the court that your opponent is farthest from.
If you can't put the ball away, hit an approach shot and follow it in to the net.
You don't have to hit approach shots hard, just deep.
Approach shots, for the most part should be played down the line. The reason for this is to limit the angles and options your opponent has for a passing shot.
Also, always follow the ball to the net. What that means is if you are right-handed and you hit a forehand approach to your opponent's backhand corner you should approach the net a couple of feet to the right of the centre line.
Your opponent will then have three options: lob, try and hit a fantastic angled backhand passing shot or try and blow one by you or through you up the line. The latter isn't much of an option especially if you approach the net with your racquet out in front of you.
If you do this you will be surprised how good your reflexes are and should be able to easily hit a forehand volley. If she hits a winner past you crosscourt it will be a truly great shot and you should applaud her for that. Ditto for a lob.
But most likely you will be able to cut off the attempted pass and put a nice backhand volley into the open court for a winner or smash one of those satisfying unreturnable overheads that bounces over the fence!