It is very easy to learn and many of my students are pretty instant riders. Some take up to a few hours. The vast majority learn quickly.
The hardest thing to overcome is having had stabilizers, if a child has had stabilizers then they will have to unlearn all the automatic behaviours that they have been doing. Stabilizers have no beneficial effect on learning how to ride.
We all balance naturally unless we have a disability like cerebral palsy or have been drinking. So we do not need to learn to balance. If when you walk along you cannot stand upright, then you have a balance problem. But I think that if you had that problem you would not be here trying to see how to ride a bike. Although if you do have a disability and would like to learn we have ways of getting every one to ride that can include special bikes.
The bike balances itself if fast enough, so if you put a bike on a treadmill with out a rider, it will stay upright. But it has to go at a minimum speed. New riders are afraid to go at any speed. So teach the student on a gentle slope, so that they roll despite themselves.
Take away fear by teaching braking first. I go into some depth about braking. If you are an adult learn to stop the thing and then you will feel safe to ride. Don’t be in too much of a hurry to ride, understand the brakes first.
If you are a parent, make sure that your child can use the brakes easily and that they work. Little hands often cannot reach the levers or do not have the strength to work them. Children have such poor quality bikes that I recommend Islabike who make good bikes for children at reasonable prices.
When braking stop while up right, this is very important. Someone new to riding will try to save themselves by jumping off the bike while riding, or leaning to one side for safety. Jumping off is not safe and leaning to one side will not let the bike balance. All the vehicle asks is that you keep it upright, so dragging one leg along the ground to keep you from falling off will never work. If you brake with both hands and then put both your feet down together you will be stopping in an upright position. Braking is so important that I would want any student of mine to be bored with it, before moving on to riding. Sit in the saddle when braking, it weighs the bike down, if you try to jump off the bike instead of using the brakes then the saddle will lift and can bash you in the back or bottom. So stop using both hands, sitting in the saddle and putting your feet down after stopping. To practice this you only need to roll 1 or 2 inches. It does not matter how far, do it 20 times, roll and stop, roll and stop. And when you stop be upright. You will know what I mean by upright when you do this exercise.
Look where you are going not at your front wheel, remember I said that you already know how to balance. You do and I would not dream of trying to teach you a clever way of being upright. We do it with our ears and eyes. ( I should imagine that our brain is in there somewhere ) We see the world and our ears have a level built into them and so together they help us stay upright. Balancing with our eyes closed is not easy, try standing on one leg with your eyes closed. In the USA the police make car drivers that they suspect of drinking stand on one leg with their eyes closed as a test to see if they are sober. When learning to ride the student is inclined to look at the front wheel, they should look where they are going, straight ahead.