Really though...
All you need from the Watchmen side is the original graphic novel.
From the DC side, you need the following facts:
spoiler (click to show/hide)
- The New 52 in 2011, launched in-canon by the Flashpoint storyline, rebooted the DC universe approximately 10 years earlier than it was set at the time. Things were shifted around in a reboot-y way, like both of Superman's human parents being killed at the same time by a car accident, but it was essentially the same DC. It did wipe out the Caucasian Kid Flash and Flash from the 90s-2000s (Wally West) and replace him with a black counterpart.
- The Superman from before the New 52 somehow survived with that reality's Lois Lane and his newborn son, Jonathan "Jon" Kent.
- Convergence led to that Superman, Lois, and Jon entering the New 52 version of the DCU right around the time the New 52 Superman died. They existed separately and in-secret during this time.
- The Superman Reborn storyline combined the pre-New 52 and post-New 52 Supermen and Lois Lanes into singular beings. Despite the new Superman taking most of his character and history from the pre-New 52 version, his parents' deaths remained the post-New 52 version (car accident at the same time.)
- Batman found The Comedian's button (from Watchmen) in the Batcave. He teamed up with The Flash in their crossover ("The Button") and discovered someone stole 10 years from the DC universe (AKA the New 52.)
- All things point to that "someone" being Doctor Manhattan.
Benji spank me if I got any details wrong.
The one thing you got wrong is that New 52 started "five years earlier" than its launch date (aside from the flashback stuff like Morrison's Action Comics and the first arc of Justice League) it became ten by the time we get to Rebirth because it'd been another five years.
Really, had New 52 not erased the Superman they then brought back and altered The Flash. (And erased the JSA grumble grumble.) They wouldn't have had to unwind the New 52. Basically every other book that hadn't gone too far off the rails went on like nothing had changed.
Wonder Woman doesn't count because her book was already in a mess, that New 52 fixed before the second writing team came in (this happened to other books...like Superman ones and Batgirl) and did stuff that everyone wanted to roll back with Rebirth. Rucka really wrote more around the Azzarello's revisions and tweak them than he did directly contradict them and revert Diana back. Even if he did totally write an equivalent to Community's whole "oh, that happened during the gas leak" explanation.
Batman's canon armor has allowed him to chug on during the whole period unchanged, even though he's the one who causes the most problems regarding the compressed time line with all the Robins, etc.
The lines about someone having "stole" ten years from them shouldn't make sense to the characters really. It could make sense to Doctor Manhattan (though he perceives time differently) and also us by what Johns probably means (the New 52), but the characters had to have had more "stolen" to leave the New 52 state at the five year point, all the events from Crisis to Flashpoint didn't take place over just five years. And we know they can't "restore" the "missing" ten years.
Though I just realized Johns might mean something like today is 20 years from Crisis. The ten years Manhattan stole were the ten before Flashpoint. Leaving the five years we didn't really see during New 52, and New 52's five years. So on a sliding time scale, Crisis was now say 1996 rather than 1986, and Manhattan pulled out 1997-2007, then slid Crisis down to 2006.
I'm using Crisis here to refer to the "time period/reboot" not the actual event in the series and its actual details like Supergirl dying, much like DC has done ever since it, Crisis was some kind of universal mega event shortly after heroes started to appear and they proliferated rapidly after. New 52 kinda used Darkseid's invasion in Justice League as its Crisis point. Batman, Superman, etc. were established as existing before it, but 99% of heroes/villains were not active, and those that did were just getting started. (The DCEU backfilled in this explanation for the lack of heroes/villains outside of Batman/Superman/Wonder Woman in
Suicide Squad, when Waller notes that Superman's public appearance was somehow tied to the rise of metahumans as a thing they have to live with now. It being tied to an unspecified major galactic or universal event known as a Crisis makes a bit more sense, especially since you can say it activated their powers or what have you. Rather than being inspired by seeing a dude fly around into half of the buildings of downtown Metropolis.)
