When bore sighting at home I use a laser bore sight like the one Dan posted above or in some cases I use the bullet-type that fits in the chamber.
With the gun unloaded and the scope mounted, you can go to the biggest room or longest hallway in the house and shine the laser on the far wall. Then center the crosshairs over the laser and that should get you on paper at 25yd or so. Works well on the backyard fence at night too, but the neighbors might get a little nervous.
Once at the range, using a rest to keep the weapon steady, center the crosshairs over the bull and let your first shot fly. If you hit the bull then you're good to move the target out incrementally and adjust your scope as needed. If your shot is off center, then hold the shotgun steady with the crosshairs repositioned over the bull. Now
without moving the weapon, slowly adjust your windage and elevation knobs until the crosshairs are over the bullet hole from your first shot. Reset and fire again. If all went as planned your scope should now be aligned with your weapon's point of impact at that distance. You can then move your target farther out and repeat the scope adjustment as needed.
It's a pretty simple method and has always had me zeroed in in three shots or less...