Trap em somewhere, throw some food inside (raspberries work) and watch the magic happen. And I quite literally mean watch it happen; you basically just have to sit there for thirty minutes while the boar is taming unless the taming area happens to be near an area you frequent (i.e. your base).

That being said, I personally don't think it's worth taming boars. Their only purpose when tamed is to be killed for their boar meat and leather scraps, and it's far easier and more efficient to kill boars out in the wild than it is to try to breed a bunch of them. Also, while Leather Scraps are needed for a lot of recipes, they are NOT a unique drop from Boars and can additionally be acquired in both Sunken Crypts and Frost Caves. There are other animals that can be tamed that actually provide things apart from their loot drops.