How do you hand lap?
The older gunsmithing books will have a description of this. Basically, you need to either cast a lap in your bore over a pilot that screws into a cleaning rod, or you upset a piece of lead wire until you have to wedge it into the bore. Coat with a very fine lapping compound and start pulling it through the bore.
You need to pay special attention to getting the slug to exit the barrel cleanly at either end - and this is where the fire-lapping is better than hand-lapping on a barrel that’s already cut to size and chambered.
Most custom barrels that are hand-lapped are hand-lapped prior to being chambered and crowned. I lop off 1 to 1.5” of the barrel blank at the muzzle end to eliminate any “belling” from the lapping. The chamber reaming eliminates the other end’s belling, so that’s not cut back as far.