[ she climbs onto the bike behind him. ] I'm going to put my hands on your waist, alright, ..? [ and after the warning she does so. ]

[ she'll explain the basics of how to work the bike, like accelerating and braking and... turning should be self explanatory, but she'll go into that too, because leaning can be difficult, especially with a passenger. ]

If you've got any questions feel free to ask, but if you're ready, then let's get going.