Whilst its been debated for a number of months now as to whether the "Arrowverse" and Snyder's Man of Steel universe should be combined, for this piece of fan fiction/ idea, we will assume that these two universes are currently on the verge of merging. But how could this be done?
The primary points of concern emanating from any conceivable connection stems from both timeline and link. In other words, is the connection already established or is it to be established? How is that connection demonstrated to the audience in a way simple and obvious enough for the non DC fans to understand.
One of the main plot elements of Man of Steel revolves around this seemingly unbeatable alien fighting with other equally as powerful foes. What is interesting is that Lawrence Fishburne states in a non-direct way that it is not just Clark's alien nature which the world has never seen but his power as well. "Can you imagine how the world would react" he says. Recently, within the Arrow universe, Barry Allen was introduced. Although he is not yet the Flash we can assume that he will obtain those powers before the season ends. If like the vigilante, the Flash is very much publicly present(i.e, he gets well known in central city), Arrow could not feasibly take place before the events of Man of Steel. Perry White is a news editor, a man running super-fast would have been a big story for the Daily Planet.
Inversely to this, if Man of Steel was a precursor to Arrow, why has there never been any mention of Metropolis. You would imagine some kind of reference to a disaster of that magnitude. Hell, we talk about 9/11 like it was yesterday for two buildings, Metropolis was decimated. In a "realistic" world, which both universes claim to represent, this wouldn't be swept under the rug. No, at this stage in Arrow's life it would be pretty unbelievable by even the most devout fans to have this take place before Arrow.
So we're setting Arrow as before Man of Steel.By extension the Flash also takes place before Man of Steel. So how can these two shows link up with Superman?
Arrow should still maintain at least another season before a connection is made. At this point the Flash will have had at least a single season to get established. To join these, both season finale episodes must end with the same cliffhanger, the arrival of Superman. This would deliver a connection directly into the 2015 movie.
Removing from the fact Superman would be all over the news, Zod makes a pretty clear and daunting entrance in Man of Steel with his "You are not alone." speech. The Green Arrow (as he should be know at this point through character evolution alone) will have finished whatever plot season 3 was about, enters the Arrow-cave and Felicity is explaining something to him with all the machines powered on. Suddenly everything goes black, and the two of them look around (Oliver ready for action, Felicity confused). Suddenly theres a screeching sound and the episode abruptly ends with the camera pointed at Stephen Amell as the "You are not alone." speech is heard and white light flashes across his face.
Another possible way Arrow could cross over with this plot is through Batman. The season finale of Arrow could possibly (but unlikely) introduce Bruce Wayne as Batman. WB probably have their heart set on having the Caped Crusader debut in a the next movie, but this joining would be a very clear and directed showing that WB are willing to put the big boys on the small screen (and take some risks). Batman isn't exactly new to Arrow either. For one, Oliver Queen does make a subtle Batman reference within Arrow when he says "You know us Billionaire vigilantes, we do love our toys". Granted this probably was meant as a clever nod whilst also a reference to his own antics. However if this were extended it could demonstrate a clear knowledge of not only the existence of Batman, but of who Batman is. Ben Affleck has already been described as a seasoned and weary Batman by the time the Man of Steel sequel comes to screen. It would be reasonable to assume Oliver has heard of him at least. Batman could show up in Starling looking to recruit the Green Arrow into his new Superman resistance force, for which he has already recruited the recently confirmed Wonder Woman.
On the Flash side of things, Barry Allen should be a first hand witness to the events of Man of Steel, but unable to interfere. The man has super-speed and would undoubtedly have joined superman to protect Metropolis. Have the final episode set in Metropolis sewer system, where Barry is incapacitated and cannot witness the blackout caused by Zod. Lets say he's drugged and tied to a chair by the guy who killed his mum. Man of Steels main battles take place over around 2 days so it would be good as a reflection episode for Grant Gustin. Just before being killed there's crashing everywhere and his captors make a run for it. The tunnel caves in and Barry is left alone and unconscious. At the end of the episode the blocked off tunnel he is facing should crumble in front of him (all blurred like he's waking up). The camera pans out to behind the chair(with his silhouette lifting its head) as Zod and Superman fly into the train station in the background.
Another way this could tie in is that Barry Allen simply doesn't want to intervene with Superman. If he stays under the radar, which in terms of keeping consistent with the Perry White issue I mentioned earlier makes sense, then he may just simply not want to blow his cover. That could lead to an interesting dynamic in the TV show as well, trying to cover up that the Flash actually exists whilst fighting crime. For this scenario, perhaps the final episode could revolve around Barry's reluctance to join the fight and expose himself.
Whilst this may all be simply speculative, the easiest way to provide a consistent universe is to integrate the TV heroes into the Man of Steel story without them being able to effectively intervene. The Flash would get involved, well because he would have been the Flash for a while. Stephen Amell wouldn't get involved due to the sheer outmatched power he would be facing (the "Arrowverse" Green Arrow is pitched as a survivor).
If you enjoyed reading this, then let me know in the comments and I'll churn out some more of my ideas on this topic.
Thanks for reading.