Summarize:

Many people are inclined to scold or punish a puppy for chasing or bothering a cat. This is actually counterproductive and could create a stressful situation, causing your puppy to become aggressive towards the cat. Instead of punishing negative behaviors, strive to reinforce your puppy's positive behaviors.  Always reward your puppy for being calm and obedient around the cat. Offer treats and praise when your puppy ignores the cat in the room. Keep a small bag of treats on hand. Whenever your puppy displays good behavior, reward her. It's better to distract your puppy than punish her if she bothers the cat. If your puppy is chasing, watching, or otherwise bothering the cat, find a distraction to keep her occupied.  Use treats, a happy voice, or a toy to distract your dog when she seems to be bothering the cat. As soon as the dog leaves the cat alone, give her a treat as a reward. You want to be very careful allowing your dog and cat to interact. Aggression can become an issue, and both animals can get hurt during a physical confrontation. If you notice signs of aggression, act promptly.  If a puppy is overly focused on the cat to the point she won't take her eyes off the cat, this is a sign of aggression. Growling, hissing, and swatting are signs of aggression in a cat. Make sure you get the animals away from each other if they're becoming aggressive. You do not want to deal with a fight. Only once your animals are totally accustomed to one another should you ever leave them alone together. Always give each animal a means to find privacy. This could be a cat-door, doggie-door, high shelves or small recess so the animals can have their space. It will take at least a month for supervised interactions to be safe. You may need longer than a month, depending on the temperament of your animals. Do not leave them alone unsupervised until they largely ignore one another.
Reward good behavior consistently. Distract the puppy if she becomes fixated on the cat. Be on the lookout for aggressive behaviors. Do not allow unsupervised interactions for at least a month.