Arrowverse viewing order: This is the official viewing order for all series and episodes in the Arrowverse.
DC's Legends of Tomorrow
Freedom Fighters: The Ray
Webisodes(most of them them are at YouTube, but you can also find them at CWtv):
Blood Rush - between Arrow S2E05 and S2E06.
Chronicles of Cisco - between Flash S2E17 and S2E18.
Superhero Fight Club 1.0 - is a promo for the final episodes of Arrow S3 and Flash S1 (arrives after Flash S1E18)
Superhero Fight Club 2.0 - is more or less a promo for Supergirl joining CW. (arrives before Flash season 3)
Constantine - watch before Season 1 of Constantine:
Modern Mystery: Tales of the Dark Arts - Who is John Constantine?
Ravenscar Institute Patient Debrief
The Flash - Stretched Scene #1 : Flash S4E06
The Flash - Stretched Scene #2 : Flash S4E07
The Flash - Stretched Scene #3 : Flash S4E09(there's also a deleted scene for this episode)
New episodes of current series:(will we addes when aired and viewung order is determained)
Arrow S06E11 and onwards
The Flash S04E10 and onwards
DC's Legends of Tomorrow SS03E10 and onwards
Supergirl S03E12 and onwards
Vixen S03E01 and onwards
New series joining the Arrowverse:
Constantine - early 2018, animated series (this will follow John Constantine from the tv-series and he is also voiced by Matt Ryan who played John Constantine in the tv-series. It will air on CW Seed as like They Ray and Vixen).
Comic books that also follow the series:
Arrow - takes place turning Arrow season 1
Arrow - Vengeance - takes place turning Arrow season 2
Arrow: Season 2.5 takes place between Arrow season 2 and 3
The Flash: Season Zero - takes place turning Flash season 1
Arrow: The Dark Archer - takes place at the end of season 3 of Arrow
The Flash: The Haunting of Barry Allen - takes place after Flash S2E07
Arrow: A Generation of Vipers - takes place before Arrow S4E07
The Adventures of Supergirl - takes place turning Supergirl season 1
Sister Act - takes place before Supergirl S1E04.
Arrow: Fatal Legacies - Upcoming - January 30, 2018 (the untold story between Arrow Season 5-6).
The Flash: Hocus Pocus - "In a timeline where Flashpoint never happened"