Playing Doctor Achievement
Stop a critical bleed-out with a bandage, first aid bandage or first aid kit
In order to get this achievement you have to use a bandage, first aid bandage, or a first aid kit to heal yourself while you are bleeding. When you make a new character, you already start off with a first aid bandage, so all that is left to do is to start bleeding. The easiest way to make yourself bleed is to loot around until you find either a glass pane or crushed glass (if you find a glass pane then you can make it into crushed glass).
Once you have your crushed glass you can select it by pressing while your cursor is over it and then you can eat it to make yourself bleed. After you start bleeding, use the bandage and the achievement should unlock after you've stop bleeding.