How to Get Someone Else's Cat to Like You
To get someone else's cat to like you, start by approaching it kneeling or sitting at its level to make yourself less threatening. Slowly extend your hand toward the cat, staying at a safe distance to avoid scaring it. Leave your hand there and let the cat approach you to sniff your hand if it wants to. Once it is comfortable enough to let you pet it, scratch it gently under its chin, and then move on to the base of its ears, and eventually to its back. If you're having a hard time getting the cat's attention, offer it treats to encourage it to approach you.
Kneel or sit at the cat's level. When trying to get to know someone else's cat, do not approach it. To appear as less of a threat, kneel or sit at the cat's level. Stay relatively still and avoid any jarring movements. Extend your finger or hand. While kneeling or sitting, slowly extend your finger or hand towards the cat, making sure that you are a safe enough distance from it that your gesture will not seem threatening. Leave your hand extended and wait for the cat to approach on its own. Respect the cat's limits. If the cat does not warm up to you, back off from it and leave it alone. Make sure that the cat understands that it has control of the situation. Pushing an unwanted interaction can result in negative behaviour such as: Scratching Hissing Running away Swatting Don't try too hard to make the cat like you. Cats can become standoffish if you pester them to give you attention. Sometimes ignoring the cat is a good way to make it feel comfortable enough to approach you. If nothing else is working, try sitting down, being quiet, and letting the cat come to you on its own time. Avoid making direct eye contact with the cat. Cats can perceive direct eye contact as aggressive. Pet it under its chin. Once someone else's cat is comfortable around you and demonstrates signs of affection (e.g. rubbing up against your leg), try petting it gently. A good place to start is under its chin, which some cats love. Let the cat sniff your finger and move its head upwards to allow you to pet it. Gently rub the spot where the jawbone connects to the skull. This should elicit a purring noise from the cat if it enjoys it. Focus on the back of its ears. Pet and scratch the cat at the base of its ears, which it is guaranteed to enjoy. Cats like being petted on areas where their scent glands are secreted so that it feels like they are marking you while you pet them. Cats are inclined to spread their scent to make their surroundings more familiar to them. This is the same aim that cats have in mind when they bump you with their heads or rub up against you. Pet its back. A back rub for a cat can feel just as good and beneficial as it does to humans. Pet down the cat's back generously, being gentle and cautious. Watch and listen for signs that the cat is enjoying the massage (e.g. purring). Avoid petting the tail. Cats tend to be sensitive about their tails, which are an important part of their anatomy. Petting a cat's tail could result in it lashing out defensively (e.g. scratching, clawing). Avoid touching, petting, or grabbing a cat's tail, particularly when you are trying to win its trust.  You should also generally avoid petting a cat's stomach. Use treats to draw its attention. As simple as it seems, food is an excellent way to win the favor of someone else's cat. If the cat is apprehensive around you, place a treat close to you, but far enough away that it will eat it without feeling threatened. Ignore the cat while it eats the treat, and continue this process over your next few visits with the cat. Toss a toy for it to chase or pounce on. To get someone else's cat to warm up to you, initiate a game with it. Toss a toy for the cat to chase, making sure that you throw it away from the cat as well as away from you. The neutral, non-threatening nature of this gesture and the curious nature of cats will make the cat likely to participate. Give it a gift. To get positive attention from a cat, bring it a gift it will enjoy. Visit your local pet store and purchase a deluxe toy such as a puzzle feeder, which you or its owner can fill with treats that the cat will slowly draw out (usually over the course of several hours). Introduce the toy to the cat and watch it have fun with your gift. 
Kneel or sit at the cat's level. Extend your finger or hand. Respect the cat's limits. Don't try too hard to make the cat like you. Pet it under its chin. Focus on the back of its ears. Pet its back. Avoid petting the tail. Use treats to draw its attention. Toss a toy for it to chase or pounce on. Give it a gift. 