Teaching a 3yr old to ride bike??

J.
Ok mommies I need some help. How did you teach your child to ride a bike? I'm a nanny for two girls and I've been here for over 2yrs. I'm having a very hard time trying to teach the three year old how to ride a big girl bike. Her Dad took the tricycle away before she learned how to ride that first! He kept saying that she needed to move on. I've been trying to teach her for months now and I'm running out of ideas. I just want to give up. She just doesn't understand what to do. How did you explain it to your child? Help please and thanks!