1. Lure boar into pen. (In your pen you probably need to make a smaller "containment" pen)
2. Throw food that a boar can eat into the pen so that it lands in front of the boar.
3. Boar will detect your presence and attack the pen trying to hit you.
4. Back away until the boar stops attacking the pen.
5. Sneak (left control) back to the pen.
6. Boar should eat and heart icons should start coming up from the boar to indicate that taming has started.

Once taming has started you can sneak away and taming will continue as long as you are within about a workbench radius of the boar and the boar does not detect you. (You can spend your time planting, harvesting, building or smelting).

The boar doesn't have to be tamed all in one session (it takes about 1 1/2 game days).

Once the boar is tamed let it out of the small pen and into the larger one. Then reset the small pen for the next boar and repeat the process. Once you have the 2nd boar tamed dismantle the small pen so they are in the same enclosure and make sure they are fed.

Stop back every game day or 2 to make sure that they are fed and you will soon have lots and lots of boars.