1. To watch (follow) someone you just click the watch button on there page. or anywhere where you see the watch button.
2. Unless the person page is private once you click watch you will be watching. If there page is privavte they will get a request which they will have to accept in order for you to watch.
3. Once you are watching you will be able to see what they post or there activite.
You can use pictures below to see how to watch someone or how they can watch you.