Learning as an adult is totally different to learnning as a child.
Use a simple bike, the right size. Small wheeled designes ,MTB style, are easier than large hybrids. Keep some plastic pedals on, and lower the seat. Make sure the gear is fairly low and the saddle is low.
First show how the brakes work, how you need to brace against the bars.
On a gentle grass slope, get rider to make big S-bends. This is much easier than trying to ride straight.Make sure the rider is wearing helmet, gloves and long sleeves/pants to provide some protection in a fall.