I learned by placing mine in a narrow hallway. The first couple times, I was bouncing off the walls like a drunkard stumbling down the hall. In short order I graduated to placing them next to the treadmill, which I use to steady myself on startup.
I've gotten proficient at riding one handed, drinking from a bottle in a cage, one-legged pedalling, standing, and can even ride no-handed for a couple minutes in one go. I've even been able to start without using external support, but I still prefer to use support to start.
Ironically, I can't ride a bleeping skinny to save my life.