We tried the traditional stabilisers method with ds1, gave up.
We lowered the seat so he could put both feet flat on the floor, took the stabilisers off & took the pedals off.
He learnt to balance in 2 x 20 minute sessions at the park. They first walk with the bike, then scoot using both legs at the same time, when he was comfortable he started off on a little hill to get up some speed then lifted his legs and balanced for quite some distance. We put the pedals back on (brought them with us and the spanner) and he was off. He was 6 (yes it took that long) as he freaked every time we took the stabilisers off.
They go from being able to pedal with stabilisers to having no idea of how to balance and how to start moving off!
Ds2 was 3, we bought him a balance bike, can scoot along like he is running but sat down, and he has learnt to balance. It took a bit longer with him, maybe 5 sessions. We have just transferred him to a normal, heavier, bike, again with no pedals on. Once he can handle that we will put the pedals on.
Scooters are good but they tire easily with them and with the bike they can sit down.
Pedalling is easy, balancing is hard.