Browntea,
Agree with the others, experienced pig keeper or vet needed to do this, and unless you're desperate get the seller to do this first.
The first centimetre or so is the sensitive part, so don't let anyone do it back to the gumline, this is what some used to do, and all it does is make the pig have a very painful mouth, introduce disease into the root system, and make your pig hate you, and a boar that hates you is not a good idea!
We had a boar whose tusks grew back into him - we use to take the final 1-2cm off using a pair of bolt croppers. We waited until he was asleep, and did whichever side he was sleeping on. We just approached him quietly, put the croppers either side without touching, and then closed them, takes confidence and speed, but gives a neat finish. As I said only the last 1-2cm, and he'd just shake his head as they do when you wake a pig by touching them, and go back to sleep. Wait until he was sleeping on the other side and repeat. We did this every 6-9 months or so as they grew back for several years.