I taught my 4 year old son to ride without training wheels in about 3 weeks, while I was at work and not even home! This was after it took my daughter until she was seven to learn the same thing. I didn't want him to take as long so I went looking for tips, and read how someone else did it. It does work!
Step 1: Recomended (strongly!)
To do this you need a child who can pedal, and who rides a bike with at least 12.5" tires and a coaster brake or hand brake.
Your kid should be pedaling and braking well,
They should wear a helmet
they need to be under voice control (stop when you say "stop")
they need a flag on thier bike also, as they only come up to the top of the hood on most cars while riding
putting a bell or horn on the handle bars gives them (and you) a quick way to give turning directions as at this age," right "and "left" may not make imediate sense. "toward the horn side" or away from the horn" will make much more sense
they need to wear sneakers or sandals that firmly strap on, flip flops come off easily and let toes get nailed by stuff on the ground. barefoot allows toes and even the foot to get crunched or cut easily.