Feeding Animals (for Grease)
After you capture the boar and go to level it up, you choose “livestock”. (Assuming it’s at level 3). Then you feed it normally with your companions until it gets the “Fat” trait.
Once it has “fat”, there should be a gear icon when you right-click the boar to manage it and there should be an option to slaughter the boar.
If your weapon(s) are too good and you one-shot, then get a crappy weapon and carry it around with you. Just use one companion to get it below 50% health, so one crappy weapon.
There is quite extensive material on boar farming within the community. The only thing I’d add is that if you keep your porkers for a couple of rests beyond the time they gain the fat status you’ll get greater amounts of meat and grease.
If you have the meat drying rack and upgrade it twice this will also produce grease every rest from meat/carcasses or oil from fish.