It takes persistence and patience to train a cat to come when called. It's best to start training your cat when it's young, but it is possible to train an adult cat. First of all, find out what motivates your cat. The most common thing is the food. Does he/she like chicken, or perhaps tuna. Whatever you will be using must be tastier than his/her normal food (dry or wet). You can have more than one flavor or type of treat in order to keep things interesting, but save these treats only for when you are calling your cat.
Next, choose how you will call your cat. It can be specific words like "come here," or "here kitty, kitty". Keep it short and simple. Also, remember to only use this phrase when calling your cat. An alternative is to use a whistle or a clicker device for calling your cat.
Try to start your training when your cat is hungry. This will make the treat that much tastier for your cat. Start out calling your cat from a short distance away, for example, a couple of feet away from each other. After your cat comes to your call, feed him/her the treat. Do this 3 - 5 times. Each time rewarding your cat with his/her treat. This will be enough for one session. Once he/she comes to you reliably (after a few sessions at a close distance), then start to do this from further and further away. At each increased distance, make sure he/she is coming when called each time before increasing the distance.
Eventually, she will come when called, even if you can't see where she is. Remember, be patient and loving with her. Watch out for boredom in your training sessions. If this happens, try switching to another treat he/she loves. Coming to you when called should always be a pleasant experience for your cat.
Training your cat to come when called is so much more than a convenience to you, it's something that could save his/her life. Should there be an emergency, or he/she becomes lost, responding to your call could save his/her life. Training your cat is an investment in his/her care that reaps many benefits.