There seams to be some crossover and I'm unsure of the order of which the episodes are supposed to be watched.
This is the order to watch those episodes . Crisis on Earth X .
Supergirl Season 3, Episode 8 (part 1)
Arrow Season 6, Episode 8 (part 2)
The Flash Season 4, Episode 8 (part 3)
Legends of Tomorrow Season 3, Episode 8 (part 4)