This will usually take place some time after the first frost, when the above ground parts of the bush appear dead. In zones eight and up, the butterfly bush will be evergreen, and can be pruned to the ground at any time.
Pruning butterfly bushes in the spring will not kill the bushes. At worst, all that will happen is that the flowers will not appear as soon, because they need to re-sprout.
Because butterfly bushes grow so fast, the blooms should reappear within a few weeks.
In colder regions of zone five, a three to six inch layer of mulch should be applied to the butterfly bush after winter cutting in order to help protect the shrub for the coming winter.
There is really no wrong way to go about pruning butterfly bushes.
The shrubs are very stable and can take a good deal of stress without any significant side effects.
Butterfly bushes will generally flower of new wood, so cutting them to the ground in winter is often times necessary for the plants to bloom.