Not knowing what your combination of Rifle and Scope are, we will take a bolt action rifle and a wind adjustable scope base.

1. In your shop or garage, clamp down your rifle with padding horizontally level and far away as possible pointing at a wall. Make the muzzle point square to that wall as possible.

2. Mount your scope base to the rifle as tight and securely as possible.

3, Take a torpedo level and place on the scope base. Make sure that the scope base is perfectly level and parallel to the rifle bore, and tighten down the vise securely.

4. Install the scope rings. Take the scope and lay it in the rings. Make sure the rings don't move and the crosshairs don't shake by spinning the scope looking thru it. If there is a shake to the crosshairs, adjust the scope because your accuracy depends on it.

5. On the wall across the shop or garage, take a blank sheet of paper or posterboard and draw a vertical and a horizonal line that is plumb and level. (Template) Fasten the paper or posterboard to the wall.

6. Take the bolt from the rifle and look thru the bore of the rifle. Line up the vertical and horizontal lines of the scope with the lines on the wall. You may also use a bore sighter light to center on the paper although the crosshairs still need to be lined up. 

7. Thighten down the scope rings and make sure the scopes crosshairs are still lined up with the wall template. If so your have found the center point.

Now your ready to go to the rifle range and zero in your rifle and scope!