How do you sight in your scope?

As far as level is concerned, get the gun level and then make sure the scope is loose in the rings. I hang a string with a small weight to make sure it is tight. With the gun level, I turn the scope until the reticle is straight with the string. That makes the reticle level too. Keep it sighted while you gently tighten the rings. They can move sometimes depending on how square the rings are.

Hope that helps.
 
At 15 yards, shoot from a rest. Adjust windage (left/right) until the pellet is in line with crosshairs. Don't worry about elevation (up/down).

Assuming 50 yard zero...

Move out to 30 yards repeat.

Move out to 50 yards. Repeat, the. Set your elevation.

Move out to 80 yards and repeat windage. Note pellet drop and hold over needed.

Move to other known distances and note pellet drop from 20,30,40,60,70 yards. Make a note pad of the required hold over/hold under adjustments.
 
Always assume that the rings are not perfectly square (they never are but they don't have to be).

Align the vertical reticle with the rifle bore (not the receiver). And then lock the scope down in the rings.

Only then should you use the plumb line. Rest the gun to indicate when the reticle is aligned with the plumb line. You only need one bubble level on the scope, not on the gun. Attache a bubble level to the scope to the mark that vertical position of the reticle and scope/bore vector.

Make sure the bubble is level for every shot (especially far shots).

Now you are ready to sight it in at any distance.
 
A trick which helps a lot is once you are on paper at any distance. Lets says your fire a shot that's six inches high and right. Now you have to do this shooting off bags so that the gun can be held perfectly still. Aim the rifle back to your original point of AIM (NOT THE BULLET HOLE) hold the aim point steady reach up and turn the vertical adjustment and turn it until it is at the level of your bullet hole then while continuing to holding the rifle still turn the windage adjustment to bring the cross hair onto the bullet strike. This will work at any yardage and in theory you've sighted the rifle in firing only one shot. It works. What you are in effect doing is lining up the barrel and the scope. You may have to fire another shot or two to fine tune but you'll be darned close. I've seen guys shoot a box of shells trying to sight in. The critical part of this is holding the rifle dead still while making the adjustments with the elevation and windage controls.