Bolt gun, semi auto, singleshot? Scoped or open sights?
For bolt actions, I rest it firmly on bean bags with the bolt out. Looking through the barrel find something at least 100 yds out and center it in the barrel opening. Without moving the rifle, adjust the scope cross hairs to the same place you picked out. That will get you "on paper" so that you can tune it to perfection. One thing I like to do after turning the adjustments on my scope is to smack the but pad with the palm of my hand pretty hard. It doesn't make the new adjustments "seat in" any better but it makes you look like you know something that other people don't know.
Another way is to start out with a target about 25 yards out. It looks too close but it will help you if the rifle is shooting way off paper at 100 yds. Get it to shoot about an inch high at 25 yds and you wont have alot to do to it out to 100; depending on what it's chambered for.
Old style iron sights are harder because of the need to use a hammer and punch to move the sights around.
I hope I didn't confuse you with my rambling instructions. Good luck.