All you really need to look out for are the Crossover episodes. Just watch Arrow until The Flash starts, then just make note of when the crossover episodes are. The first few seasons of all the shows are great, at least until Supergirl starts lol. Supergirl gets better as it goes, compared to the other shows.
EDIT: And Constantine is completely separate. You dont have to follow a watch order for him, unless you just really really want to lol. He didnt become an actual DC CW player until he joins the Legends.
"An immortal Egyptian; aliens; parallel-Earth Nazis; I have no idea why I'm not even surprised at this point." —John Diggle[src] A list of crossover episodes which involve characters from multiple shows within the Arrowverse. They are arranged chronologically, grouped by season. Main article...
arrow.fandom.com
Looks like the first actual crossover is the Season 1 opener of Flash, with the Season 3 opener of Arrow.