Chivalry 2

The way that I found was easiest for doing this was on the Dark Forest Map for the Mason team. You start by pushing these wagons along and one of which will resupply you (Including bandages). Now to actually taking damage without dying.. you can do this one of three ways with the 3rd being the easiest.

1) Get into a fight and survive it one way or another
2) Poke a teammate with your weapon a couple of times hoping they might stab you
3) Have a friend of yours join on your team and have them hit you so you can heal
Edit: 4) As mentioned by a commenter you are also able to simply burn yourself with nearby wagons or anything else that may be on fire

So each time you take damage and you're safe press (by default controls) which will heal you as long as you have one. You can then proceed to go to the supplies location I have mentioned previously or any other that you may find and press and hold to resupply. Now repeat the healing and resupplying without dying.

For people that skipped on the tutorial.

PRESS UP ON THE DPAD, to heal yourself.

You can damage yourself by standing on top of a torch, d pad up heal yourself, look for an ammo box to resupply yourself with a bandage and do this two more times. (3 times in one life not one game, ONE LIFE!)