1. Accept the humbling possibility that many things you learned and think you know about killing whitetail bucks (in general) simply may not apply with old deer.
2. Slow down. Scouting, hanging stands, observing sign, enter/exit, monitoring activity, and studying their behavior. You'll start to pick up little things they do differently that you can key on.
3. Wait until the right time to hunt him...when he's most vulnerable. You'll need every possible advantage to kill old deer consistently and even though you found him in August may not mean opening day of bow season is the best time to kill him.
I choose these 3 in part because they were personally, my most difficult challenges. C'mon, how tough can killing a mature buck be? Boy was I WRONG.
Totally completely different animal.