How do warlocks get into a pact with their patron? I want to multiclass my rogue but I don't know how that works into any story.
Are there any published official material (or Unearthed Arcana) that guides the DMs on how to integrate backstories for warlocks? Or any recent WotC novels that describes such stories?
There are two things to consider here: (1) the player's demand; (2) the character's demand. If you wanted to multiclass to a cleric, you could easily say that your character is going through an existential crisis and then head for the nearest temple for guidance. But in the case of a class like warlock, the answer might not be so easy if your character has never met someone who is a member of that class.
Since the warlock-patron bond is highly setting dependent, it is best talking to your own DM for specifics. Here I will try to offer a few examples from the Forgotten Realms setting, as it is the "default" setting to which a large number of the 5th edition sourcebooks regularly refer.
If it is your demand to play a warlock, but your character has not encountered any warlocks or possible patrons before, you need to declare to your DM that you want to multiclass and she might come up with an event that will naturally push the "destiny" of your character towards becoming a warlock. For example, you or one of your party members might accidentally summon a being from the outer planes. This is the story of Farideh from the "Brimstone Angels" series; she is a tiefling who met her patron when her twin sister's spell to summon an imp went wrong. A short summary, as well as Farideh's 5th edition stats are available here.
If it is your character's demand to become a warlock, because he has heard about or encountered warlocks before, then you should tell your DM what he (your character) is actively doing for this. He might try to find ancient tomes in libraries, or seek hallowed or desecrated grounds for praying, or get in touch with known warlocks, or groups that are likely to have warlock members or associations which might be in the service outerworldly patrons. Examples could be the Host Tower of the Arcane, the Cult of the Dragon, etc. Maccath the Crimson from "The Rise of Tiamat" adventure module appears to be such a warlock according to the FR wikia.
FR wikia offers a couple more of warlock backstories under the Warlocks (5e) article.
Mike Mearls, one of the Lead Designers for 5e recently did a podcast interview which should clear up a lot of your questions, though it really all boils down to, “it will be different in each campaign”.
The relationship between warlock and patron, and the details of how they communicate, enter the pact and enforce it are left intentionally vague.
Mearls says:
It really, in practical terms really just depends on that you want to do with your campaign. In my campaign the warlock has a pact with an actual villain that they're dealing with and so that's actually something that they have to, not exactly work around, 'cause I'm not using it to hose the player or take power away or anything like that, but that's just part of the campaign background. In your campaign it's just a matter of how much do you want to use it. It's like a god. Do you want the deity to be giving proclamations and giving quests and being very active or do you just think that's more of a background element. It's like being a member of a guild, it's just part of your character and you make it what you will.
There is a lot more in the interview and it’s really worth listening to, but no concrete rules are given, because none are assumed. The nature of the pact is heavily dependent on the campaign world. You need to consult your DM, as it likely will tie very heavily into whatever world-building they might have done on this matter.
It's really a matter of making up a story, which is what we're all doing anyway. You'll need, of course, to collaborate with your DM on it. And it can be extremely complex, or very simple, or somewhere in between.
Old Ones are enigmatic and just do weird things, so any number of random events could serve. Fiends and Fey both are known to play tricks and entrap people and use them for their own purposes. Maybe the character doesn't even choose, or initiate the pact; maybe the Fiend or Fey chooses the character, and sends an emissary to lure/trick/convince him to enter into it.
For example, in the case of one of my characters (who started as a fighter), he had rolled a trinket in his starting gear, which happened to be the "small idol depicting a nightmarish creature that gives you unsettling dreams when you sleep near it". Since I wanted to have a pact with an Old One anyway, that made for a simple hook. At one point my character decided to spend a long rest meditating on the figurine. It came to life, burrowed into his body, and fused itself onto his spinal cord, and that was his link and became his pact.
The DM was fine with that, and voila, the origin story of a Warlock!
