I second Deb's suggestion, scooting on the bike with one foot on a pedal, leg vertical, should be a good way to safely get a feel for the balance you need. Since you have weight only on one foot and not on the saddle, you'll probably feel a bit wobbly to begin with until you get used to steering with a little weight on your hands.

Next could be starting with one pedal horisontal, clipping in and starting with a downward push before just rolling a bit and stopping. The bike will feel more stable when you start with a push and a bent knee rather than just scooting.

I usually start with the right pedal just past the top, click in and push down as I kick off from the ground with my left foot. That gives me enough momentum to sit on the saddle and roll until I can clip in the other foot. Only place I have trouble doing this is up steep hills. I would feel very unstable trying to balance on the saddle before starting!