Like many of Diablo Immortal's events in the zone the players won't be able simply walk up to the Haunted Carriage D2R Items whenever the mood hits you. The window for participating in this event is extremely tiny.
The Haunted Carriage shoots at Ashworld Manor every Tuesday and Saturday between 12 pm. The times are 8:30 pm, and 10 pm (based on your local server time). It's usually recommended that you arrive at about 10-15 minutes earlier in order to ensure you don't miss your shot however that's more of a general suggestion rather than a obligation. Whatever the case, as that you're in the nearby vicinity of the carriage, you'll be shown an on-screen announcement that informs you the event will begin and asks you if you'd like to participate.
When the Haunted Carriage is born it'll follow a pre-established route to its eventual destination. In addition, you can just walk up to the Haunted Carriage when it's en way to its destination (or just sit and wait until the end of the road), but you're really more likely to follow the carriage as it rolls along instead of only taking part in a tiny portion of the celebration. For reference, here's a diagram of the carriage's route across the region (map taken from Wowhead):Once you've found the carriage, it's time to be a part of the event itself.
The Haunted Carriage follows its designated route the carriage may be confronted and attacked by guards. Those guards' locations are identified by skulls that appear on this map. There's probably D2R Ladder Items Buy a tiny difference in where exactly the guards' spawning locations, but you'll likely encounter them in that general area.