Can you borrow a bike that will fit her?
Basics for starting to learn.
Start with the saddle set so that both feet are just flat on the ground when seated.
Cover the brakes at all times.
Apply brakes to get on the bike, and at all times when not moving. It makes the bike seem much more stable and inspires confidence.
Having previously set the bike up in a nice low gear, she gets used to bringing the starting pedal to a 10 o’clock position.
Aim for setting off and just getting the other foot on the other pedal. So half a pedal stroke from the starting position, a gentle incline can help, as above. Then coast, gently apply brakes to a controlled stop.
Do this a few times, no need to rush. You walk alongside to offer advice (“Cover the brakes”) and support.
As soon as she stops remind her to get ready to go again, brakes stay on, pedal to 10 o’clock.
Then gradually increase the distance and number of pedal revolutions. Until you get to jogging pace (“Cover the brakes”) and the bike starts to self stabilise. Practice coasting with the starting pedal at 10 o’clock so when she stops, she is ready to start again without have to adjust anything.
She should now be able to ride. Tennis courts are good, just enough space for her to ride around you so that you can offer advice (“Cover the brakes”) on what to do.
Complete beginners tend to steer away when they start to lose their balance. Walk with the bike and show her how when you lean the bike, what happens when you steer into or away from the lean.
They also tend to target fixate, “Look were you want to go, not at the (Insert object, lamp post for example).” Something like tennis courts are good as they have the line markings to follow and nets to avoid.
Explain gears off the bike not while she is riding.
As confidence grows, adjust the saddle height to a more efficient position.
Have fun.