Ive read both 5/3/1 and 5/3/1 for powerlifting and did the program for over a year. My standard assistence template is as follows.
Squat day
Squat 5/3/1 or 3/5/1 depending on things are going
Hamstring/Lower Back Lift-Usually a variation of a Goodmorning here, not heavy like low bar arch back goodmorning with 135 for 3-5 sets of 10, sitting back as hard as possible makes this very painful.
Unilateral Quad work-Usually lunges, unilateral leg press, or split squats, 3-5 sets of 8-12
Abs-Standing Rope Crunches 3-5 sets of 10-20
Bench day
Chins warm-up, idk how many you can do at a time. usually 3-4 sets.
Bench+Chins (Chins inbetween all sets of bench including warm-ups, depends on how many reps you can do, i was doing 50-70 chins a session)
DB Work-Inclines/Flat 3-5 sets of 10-20
Rows Variation 3-5 sets of 8-12
Tris+Upper Back-Pushdowns+Face Pulls for 100 reps pushdowns and face pulls
Deadlift day
Deadlift
Unilateral Quad work-3-5 sets of 8-12
Hamstring/Lower Back-3-5 sets of 8-12
Abs-3-5 sets of 10-20
Standing Oh Press day
Chins warm-up, same as bench day
Standing Oh Press+Chins (same as bench day)
Dips-3-5 sets of 6-20 (examples being bw for total reps, weighted, and banded rotate in 3 week waves)
Row variation 3-5 sets of 8-12
Tris+Upper Back-Oh Ext+Pullaparts 100 reps of each
The lifts picked are examples id pick the ones that help you the most.