do it with a friend. As in, go to a quiet path or road, with a few hundred metres of flat or slight downhill, and have an experienced cyclist with you to give you a bit of confidence and (this sounds daft, but....) tell you when to unclip.
Make sure the pedals are clean, and that the spring is lubricated. Start by clipping in and out half a dozen times with your non-standing foot while stationary. You might ask your friend to loosen the pedal a bit if this proves difficult. Then push off with your non-standing foot clipped in, go for twenty metres or so without clipping in your standing foot, turn the pedals a couple of times and then come gently to a halt - unclipping the non-standing foot. Do this half a dozen times. Re-assure yourself that your standing foot has not accidentally clipped in by moving it clear of the pedal before you come to a stop.
The big one - push off with your non-standing foot clipped in, attempt to clip in your standing foot. Don't panic if you can't find the clip first time - just come to a stop, moving your standing foot clear of the pedal before you come to a halt. When you succeed in clipping your standing foot in, unclip again after a couple of turns, move your standing foot clear of the pedal and come to a stop. Do this another half a dozen times. After a couple of goes, leave the non-standing foot in the pedal for a bit, just as you would if you were at a traffic light.
Try the length of your street or path, but have your friend follow you and tell you to 'unclip' thirty metres or so shy of the end. That gives you a little bit of a comfort zone - again, move your standing foot clear of the pedal to give yourself that bit of re-assurance. Once you think you've got the knack, you can take to the road...
But have someone behind you when you make your first (and, perhaps, second and third) trip on the road - to shout 'unclip' as you approach a stop. Again, this sounds silly, but it helps.
That's how the love of my life started her clipless career, and she's never had a clipless moment. Not one.