well there are several ways smiths do it. the most common is where a special rod with a lead blank (look like a little lead bullet with driving bands) is ran through the barrel with a polishing compound. this way works well because the lead takes the form of the rifling. aftermarket barrels that get a lapping are slugged (lead is poored down the barrel) and then the lapping compound is applied. this way produces the best results but is hard to do. these ways are forms of hard lapping, lapping where the material holding the compound is near the same hardness as the material being lapped. soft lapping is where something like a clothe, rubber blank or clay is used in palce of a harder material and is done basicaly the same way.
if your barrel is already broken in it might not benefit from a lapping, it might actualy reduce accuracy. things like muzzleloaders or shotguns can be lapped without loosing accuracy because the projectile expands to the rifling rather then being engraved. soft lapping can be done at home with jb bore bright, which is the finest rifle barrel specific lapping compound being made and it rarely ever decreases accuracy but it takes some time for it to increase accuracy. it is used on a patch and ran though the barrel for about 120 passes through. you can use it about once a month but its best to only use it every 20 shots when the barrel is being broken in