I used a laser bore sight on my AR-15 and centered the scope. That was enough to get the rounds on the target. Then take it to the range for fine tuning. If you are in Montgomery County, I can help and you can use our range as my guest.
Yep- zero to understand the relationship that occurs when the line of aim and the line of departure are parallel both vertically and horizontally to the bore until the bullet strikes below point of aim a distance equal to the height the rear sight is above the bore plus the amount the bullet drops to gravity at a specific distance.
To get it on the paper, take off the upper (assuming AR). Remove BCG.
Set up the upper on some bags, so that looking through the barrel, you are aimed at some mark, at least 25 yards away.
Adjust red dot to point at the same mark.
At the range, fire one round at a target. I normally use 25 yards. Pull the upper off again, and prop up, so that the red dot is aimed at the target. Without moving the upper, adjust the red dot to point at the bullet hole.
If you want a 36 yard zero, adjust the red dot to point about 3/4 inch BELOW the bullet hole. This is for 55 grain bullet at 2900 fps.
Shoot at your desired Zero range. You should be very close.