It is all about being creative. Treat it like a puzzle, where you look for clues which will lead to other clues and eventually you will find what you are looking for. For example:

• Sometimes people decide to use a username on different websites (or a very similar one), use this to find their activity on other websites and you might find more information that could be useful to you.
• Anything helps. If they upload a photo of food they ate outside but didn't mention the name of the place, then you can search what places in the city serve that type of food. Stuff like this is very important for the next point.
• Maps are very important. You can create a map based on the locations they have visited and start narrowing down the area where they live at. Social maps are also important, start finding out what that person's social circles look like.
• Sometimes the easiest way is to gather information about other people close to them. You will eventually find a friend or a family member that has weak online privacy and will help you with your search.
• It is good to invest in one or a few fake online profiles. Especially a female one, as they get contacts really fast. You can use these fake profiles to start infiltrating their social circles, most of the time guys will accept the invitation of a random sexy girl. You can also customize your fake profile to have similar interests to your target (music, movies, games, events, etc), this makes it easier for them to take interest in you.
• Practice meeting random people on the internet if you are unsure on how to get them to trust you. Learn what pushes people away and what makes them more interested in you. With experience, you will know exactly what to say so they don't suspect you.
• And finally, keep your story straight. Don't mix your fake profiles or online accounts and never reuse them.