What type of bench press technique should I use if I broke my ulna?

None until healing. You should get current xrays to confirm healing and clearance from your md before resuming bench press. Once healed you can resume your usual technique (except in extenuating circumstances), just be smart about gradually increasing weight/reps/sets/intensity--you won't be 100% and could risk injuring other structures if you do too much too soon. See an orthopedic specialist in your area.