When I bought my double rifle, I wanted to revive a old concept, marksmanship. Here is what I have done:

0: Learn correct posture. A shotgun coach can teach you this. Start shooting trap and/or skeet.

1: put a 3" target up in you living room, get some snap-caps and practice mounting, aiming and squeezing your trigger. I call this "trigger memory"

2: work up some pet loads for range work.

3: if you can, start at 25 yards with a 200 yard target, focus on the target and shoot with both eyes open. Don't aim! This is called instinctive shooting. Work your way down to a 6" target.

4: for 50Y or longer I focus on the target and let the blurred front sight pass across the target once or twice and try to squeeze.

5 The most important part is to relax and have fun.