Prune out all but the best trunk and cut back the limbs by about 1/3rd.
Our if the trunks are separate plants you can separate them into separate containers, and prune as above.
Next year you do more pruning to start shaping it into the form you like.
It can be loosely trimmed to arch over into a weeping form, or trimmed into a more formal topiary ball. You can trim throughout the season to maintain the size and shape you want.
Pruning should be done in late fall or early spring.
I find that this willow grows rapidly in the growing season.
