You wouldn't ordinarily prune a Nandina at all - they are very compact shrubs and won't generally get much taller than 4ft in most situations anyway.

The cultivar N. 'Firepower' only reaches 4 feet, but Nandina 'Richmond' reaches 8 feet - hopefully, you've got the first one. Pruning recommended is to remove old and weak shoots in spring to ground level each year.

If you have one that's already too tall, you can remove the tallest 1/4 of the stems down to ground level each year, until it is the right height. You might want to feed a little extra during this process to keep the plant vigorous. Once it's the right height, simply remove any stems that get too tall as needed.