While riding along, stop and jump quickly. Then go back and do it again. After not long you’ll be able to do it in a fairly smooth fashion, to the point when you’re not really stopping at all; the pedals stop turning very briefly as you begin your jump but because you’re moving forwards the jump isn’t just up but forwards too.
Rhysling has a picture somewhere explaining Dan Heaton jumping onto a car bonnet*. Start the way you mean to go on…