Harvest the tops of branches bearing catkins.
Remove any dead branches.
Then cut one-third of the oldest branches back to the ground. The oldest branches are the gray-colored ones.
Next determine where the newest (brown-colored) branches are — the vigorous new growth coming from lower on the main stems. The tops of these branches will serve as gauge for your remaining cuts.
The remaining cuts will be made on the branches the tops of which you just harvested for their catkins. Make your cuts back to level determined in step #4.
Repeat this process for three years in a row. At this point all of the oldest growth of the pussy willow shrubs will have been removed — you have a “new” shrubs.

Of course, you will also be pruning with an eye to shaping the pussy willow shrubs to suit your tastes. Use sharp anvil pruners and make your cuts above nodes. Cutting above nodes that grow along the outside (i.e., furthest from the center of the shrubs) of branches is most effective. An offshoot from the outer part of a branch will grow outward and is less likely to cross over other branches. Branches of pussy willows that are already crossing should be removed. They shade each other, reducing the number of catkins.