Answer: You can prune your forsythia and quince once they've finished flowering. Both are vigorous and won't mind being cut back quite hard. Kerria japonica can be pruned at any time, but waiting until autumn will allow you to enjoy the summer bloom. To completely renovate overgrown shrubs it's easier on the plants to do so in stages, removing only 1/3 of the live wood in any one pruning session. Plan to take 2-3 years to reduce the size of your plants and get them back into shape.
All the plants you mention are easy to propagate through layering. Find a lower branch or stem that you can easily bend down to ground level, nick the side that comes in contact with the soil, hold the wound open with a pebble, then bury the wounded portion and anchor it in the soil with two "U" shaped hooks (from a wire coat hanger). Roots will form at the site of the injury as long as you keep the soil reasonably moist, and within a year you can separate the newly rooted stem from the parent plant.
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