After your mounts are installed and hardware is tightened, install scope rings on the scope itself and leave loose enough to adjust their position. You will need to be able to rotate the scope and to move it back and forth after mounting on the rifle. Make adjustments as needed so that the horizontal line on the reticle is level. You will also need to adjust the eye relief on the scope. While keeping the gun in a comfortable shooting position, move the scope forward or backward until there is no black ring around the objective. It’s also important to make sure you have enough eye relief that the scope will not contact your face due to recoil.
You’re not going to be shooting at the target, just lining up the sights, so this is a perfectly fine thing to do indoors, if you’ve got the space for it. Just practice good gun safety, as always, checking and double checking to make sure that your gun is unloaded before you try to bore sight it.
What you’re looking for is having a perfect sight picture when you’re looking through the end sight at the appropriate cheek-weld and sight picture, while at the same time you could look through the scope at the appropriate distance and be lined up with the exact same spot. It’s very difficult to do this without a gun vise or some other kind of secure stabilizer, so you can make your adjustments without holding or moving the gun.
