The Boring But Big is probably the most popular, the easiest and the most widely talked about assistance protocol for 5/3/1. There are numerous ways to do this and hereis one of the worst/best. Like always AND especially for this variation I highly advise you to start with a weight that is too light. Your body will thank you.
I really only recommend this variation for the squat. There is something to be said about doing some insane lifting/volume for the squat every so often. It will build muscle mass and mental mass; two things that are often missing in all of us.