Core answer: get the DLC to actually appear first, then start the Joker/Harley story, then do not waste your first Mayhem Runs. The number one confusion: Joker and Harley do not join the main Gotham free-roam roster.

1Get the DLC to Show Up

2Play Order

Do not expect to drop Joker or Harley into the main open-world Gotham. They are mission + Mayhem only. Vehicles exist because you steal hero cars during runs, not because they roam the full map.

3How a Mayhem Run Actually Works

Combat is ranged hit-and-run, not Batman melee. Joker uses the Paintball Pelter and extendable boxing glove. Harley has the Fun Gun (confetti/bubblegum), hammer, and hyenas Bud and Lou. Base-game takedowns are gone here. Keep moving.

4Key Hideout Decisions

Spend studs onPriority
Movement and survival upgradesFirst - finish runs instead of timing out
Joker-Mite shop equipmentSecond - "Well Equipped" needs every piece
Hideout expansionsThird - "Base of Operations" for fully expanded
CosmeticsLast, unless you only care about skins (BTAS Harley is a common goal)
Crew: rescue Arkham inmates on the island, have the Henchmaster train them, then assign a custom crew you can summon. Targets: 3 recruits (Welcome to the Crew), then 10 summoned at once (Full House). Harley's hyenas also open areas, fight, and help loot shops.
Pop-up shops vs Joker-Mite: buy every power-up once from pop-up shops for the Bargain achievement. That list is separate from Joker-Mite equipment.

Common Mistakes to Avoid

Launch Checklist

Videos

Direct walkthrough for getting Mayhem Mode to appear on PS5
Official trailer showing the mode's fantasy (not a how-to)