1. Remove the bolt as you would when cleaning the rifle.
2. Find a way to securely position the rifle on a table or other steady rest (sand bags, a rifle rest, etc.)
3. Sight down the bore to a distant object. I usually set up on my kitchen table and sight down to my neighbor's bright white mailbox about 100 yards away. Maneuver the rifle so that your chosen object is directly in the center of the bore.
4. Now look down the the scope and without disturbing the rifle, center your crosshairs on this object.
5. At the range, shoot a three shot group at 25 yards. Then steady the rifle with the crosshairs on center of target, and again without disturbing the rifle, adjust your scope so that the crosshairs line up in the center of this group.
This is the method I use. No expensive bore sighters; no fees to pay. It ought to put you awfully close at 100 yards.