Write an article based on this "Send them a message. Invite them to meet. Plan a fun event."
If you find your friend on a social media platform, consider sending them a message. This is a very common way to contact a person these days, so don't feel too shy about it. Remind them who you are (be wary of assuming everyone always remembers you) and tell them that you are interested in catching up some time. But remember that Facebook has a feature that filters out messages from people not on your friends list and sends them to a separate "other messages" folder. It may take a while for your friend to see the message because of this if you don't add them as a friend first. Tell your old high school friend that you would like to meet up to catch up and see how things are since you both left high school. Invite them to coffee or lunch, or to your home if you feel comfortable with that. If your friend lives too far away to meet up, consider calling them or Skyping with them. Or you can just catch up with some messages or emails back and forth. Consider planning an outing for you and several of your old friends from high school. This could be a fun gathering for all parties involved. Ask some of your other friends from high school if they would like to meet up and then mention it to your old friend once you have found him or her. Making it a group event may also take away some of the pressure when you see your old high school friend for the first time.