First, be sure the firearm is bore-sighted, and you do NOT need a bore-sighter to do this (If you use a bore-sighter be sure it is removed from the barrel BEFORE you fire the gun.) BE SURE THE FIREARM IS UNLOADED. Look through the chamber end of the barrel, and center an object in it that is 75 to 100 yards away. Lock the barrel in a shooting rest or vise (be careful not to damage the barrel) keeping that object centered in the barrel. Adjust the sights so they are lined up with the object centered in the barrel. You have just bore-sighted your firearm.
Second, fire a two shot string at 50 yards. UNLOAD THE FIREARM AND LOCK THE ACTION OPEN. Check the target for nice, round holes from the slugs. Lock the unloaded firearm in the shooting rest so the sights are lined up where you aimed for the shot. Adjust the sights to center the POI.
Third, fire one or two rounds to check for zeroing. Make any fine tuning adjustments for windage and elevation. Adjust the elevation for the POI you want to have at 50 yards (2 inches high, etc.).
Fourth, fire one or two rounds at 100 yards to check elevation. Do NOT adjust windage. Slugs are very prone to wind drift. It is recommended the windage to be set at 50 yards, and you are sighting in under good shooting conditions (little to no wind, etc.). Use the number of clicks need at 100 yards for your optical sight elevation adjustment.
Fifth, fire two rounds at the maximum distance you will be shooting to check for proper elevation (2 inches low, 4 inches low, etc.)
Sixth, test the firearm with a cold barrel on another day with favorable shooting conditions by shooting a two or three shot group at your maximum distance. If adjustments are made, fire one or two more shots to verify proper adjustment.