Summarize this article:

In preparing to go, the first person should hold the handlebars firmly and stand, both feet flat on the ground, while the other person mounts. It is much easier if the person in front has a backpack. This is difficult, but you have to keep the bike as straight up and down as possible, or else the takeoff is nearly impossible. It hurts. They have to sit on one side of that particular body part.

Summary:
Have the more experienced cyclist mount first. The other person should then sit down on the seat and put their feet on the rear axles while holding onto the first person. The person in front should put their foot on the pedal and gently take off. The person in front should stand up while pedaling, but if their legs get tired, and they're riding a man's style bicycle, they can sit on the tube, but they can't sit directly on it.