Hi there and thanks for the post.
This is one of the hardest problems to solve and many people fail.
It is very hard to catch your dog in the act so you can reprimand them but lets go over some ideas.
Start with obedience as this is the most important issue to deal with. Work with your girl for 10 minutes twice a day in a quiet secluded area on the basic commands of sit, stay and come.
Having these under control is the easiest way to stop any problems.
Now getting a cat to walk around so you can help train her is way too far out.
But what we can try to do is use a cat cut out on the fence or on a stick to simulate a cat moving around.
So move this around. If your girl gets out of control and starts getting aggressive either barking, growling, rushing or anything she needs to be told off. You need to use a loud GRRRRR as well as a squirt with a water pistol or throwing a blanket over her.
After she gets over it ask her to sit and praise her if she responds.
Use the same process if you see that she sees a cat and gets aggressive. Another idea is to put her on a long line so that you can control her. As she gets better behaved with real cats or the cut out give her more and more line. Every few days she improves and doesn't react then give her another metre of line.
Good luck with her. This is a very frustrating problem and please feel free to get hold of me whenever you need help
Todd