I assume you are starting on level terrain?

Clip in with one foot while astride the bike.
Do not sit on the saddle.
Raise clipped-in foot to about the 10 o'clock position.
Push down hard with that foot to begin pedaling and lift your posterior into the saddle.
KEEP pedaling with the clipped in foot and just slam down the other foot onto the pedal.
Pedal a few more strokes to get your momentum and speed going.
Clip in other foot when momentum and speed have you moving safely forward enough to coast a second.

If you can learn to pedal one-legged with the clipped in foot, you'll find that you can get your speed up a bit faster and will help you learn uphill starts later on.

Sooner or later, your unclipped foot will learn to find the pedal as you slam it down and clip in all on its own.

I personally think wobbly starts are due to people thinking they have to get both feet clipped in immediately.