Arrowverse viewing order:
[pending on update...]
This is the simplified list for easy viewing order for Arrowverse!
Webisodes (you can find most of these videos at YouTube or at CWtv website):
Constantine has a cameo in Arrow S4E05. I do recommend you to watch this series, but there's no must. But he might get back into the Arrowverse one day. (and I also would recommend to watch it in this order: 1, 6, 2-5, 7-13. Episode 6 was shot to air as the second episode of the series)
I have added Supergirl Season 1 before the crossover episode to the list. You can watch it basically whenever you want, as long as you watch it before Flash S2E18 or after Flash S2E18 (I personally prefer to watch it before Flash episode, but both ways work). Also S1E04 and S1E05 were aired ouut of original order. Episode 5 should be viewed before episode 4
Constantine: City of Demons: So it appears that Constantine: City of Demons is not a continuation of the Constantine series and so it is not included in Arrowverse. Instead it's in the same universe as Justice League Dark (so it's in DC Animated Universe). But for now I'm going to leave it in the list, as it still explains things like "Newcastle" and etc. But they may get connected, as Constantine will becomes a regular on DC: Legends next season. So we will wait and see.
Crisis on Infinite Earths: Starting with Supergirl part 1 - Sunday, December 8th, following with Batwoman part 2 next day on Monday, December 9th, with The Flash part 3 on Tuesday, December 10th and then starting the new year off with Arrow part 4 Tuesday, January 14, 2020, right after followed by the last part of the Crossover event on the same day, Legends of Tomorrow part 5 ending the Crisis on Infinite Earths crossover event. This event will also tie-in Black Lightning into the arrowverse universe among other things.
There will be a Arrow spin-off series following Mia (Katherine McNamara) also Canaries Laurel Lance and Dinah Drake will be part of the spin-off.
Earths / Connected Shows
The Flash (1990) - earth 90
Supergirl -earth 38
Freedom Fighters: The Ray - earth X
Birds of Pray
Comic books that also follow the series: