Warning: before we start, this guide contains spoilers for both chapter 3 and 2, so click off it if you haven't played them, now onto the guide.
Getting the Bone Guide
Nobody seems to care about buddy boris, they're all cooped up in "Tom" Boris asking if he is Thomas Connor, blah blah blah, this guide'll teach em' a lesson! So. The first step is to start chapter 3 Ya dingus! Now proceed on over to the toilet, there are two doors, one opens and one doesn't (this'll be important later) if you jump and try and look over the locked door you'll see some Bacon in the toilet (I don't even know why the hell there's a Bacon Soup jar in the toilet, Boris, you nasty) now leave the toilet, umm, I told you to leave the toilet, what are you doing in there? Ugh! What's that smell, wait a minute, Oh no, in public? Really? Boris! Boris?! Anyways back to the guide, you need to go out of the toilet and proceed into the main room where boris is sitting, now you need to turn left, you see a door? Go towards it, you'll hear Henry say that the lever's missing, now go back to boris, interact with him (E on keyboard, square/x on consoles) you'll need to feed him 3 Bacon Soup, first one can be found put on a shelf in the main boris sitting room or whatever, second one can be found in the chest in the room you wake up in plus there's another shelf with Boris' most valuable belongings which just so happens to house Bacon Soup, now go to Boris, look at him square in his eyes, his horrifiying demonic eyes, anyways turn right and you'll see a stove, interact with it to cook the Bacon Soup, after it's done cooking give it to Boris, an he'll give you the lever, then go to the door, put in the lever and open sesame!
Getting the Bone
After you go out the door, it'll show you that you can never return to the safehouse, do not go out of it and instead go back into the sanctuary/safehouse, and go to the toilet and consume the Bacon Soup, then go even further back to the room you woke up in, right next to the chest will be a bone that once you interact with will disappear, but do not fret or rage, go to Boris, interact with him (he won't glow like you usually do like the bone) and bam! The bone is in his mouth (what did I just say lol) you'll get an achievement for this.
Once you fight the Brute Boris boss in Chapter 4, he'll have a bone in his mouth, indicating that this is the same Boris from Chapter 3, so sorry to people who're saying that the Boris we see in Chapter 4 is different from the Boris in Chapter 3.
When the lift crashes in Chapter 3 ending the bone'll still be there in Boris' mouth, that bone is the strongest bone I've ever seen!