Lapping would be rotating the needle in the seat
with an abrasive compound to get the surfaces of
the needle and seat to be identical for the best seal.
For this to be successful alignment is critical so it
would be best done with the float bowl cover on.
Having a piece of tubing to slip over the needle
is helpful as to pulling the needle into the seat
while rotating it back and forth. You will likely
need to remove the compound several times and
repeat to obtain a good seal which should keep the
carburetor from running over for at least 15 minuets.
Its unrealistic to expect a total seal so you need to
shut off the fuel when the motor isn’t in use.
Its possible to have a motor sit for hours before the
float bowl runs over but they wont make it overnight.
Your fuel shut off needs to be working also.
If in fact someone has altered the seat lapping
wont be able to undo that. Corrosion of the aluminum
over time makes the seat a little rough. Lapping is to
make the surface of the seat smooth again.