Build a pen with an opening and make sure you have a Workbench nearby. Have the fence piece ready and equipped with the hammer. Aggro a boar so it starts chasing you. Lure it into the fence opening. Close the fence and feed it mushrooms or berries. The boar will be tame in a few in-game days. Do this nearby with another wild boar, open the pen and extend the fencing so they can mate.