I used to work in an engine reco shop years ago...
These days most reco shops just reface the valves and recut the seats.
The 45° valve angle and seat angle are about 1° diffent to each other.
They are supposed to seat themselves without any lapping. So-called `synchro seating'.
That said, I've seen enough dodgy valve and seat cutting that I still lap them in by hand afterwards, to be sure they are concentric.
This is done with a lapping stick (wooden dowel with suction cup on end) and some fine valve lapping paste.
You put a light smear of paste on the valve face in about 6 spots, drop it into the seat then use the stick in a to and for motion, give it about 10 or so oscillations.
Then lift valve out, wipe it and the seat and inspect both. Repeat if necessary. When you have a dull grey contact area 1mm or more wide on both, move on to the next one.
If the contact is not quite all around, either use some coarse paste to give metal removal, or preferably take the head back to who did it and have a whinge...
Try not to get any paste and grit on the valve stems......
When finished degrease head to get all the grit off the seats, and the valves. Then assemble.