All solutions are from my playthrough and experiences may change, if your playthrough does not match comment why and I will edit
*** Spoiler - click to reveal ***
Start off the chapter at a hostage situation on Gotham Bridge
Have a chat with Waller, eventually you will talk to Harley
John will want to join
Tensions rising, you will have the option to trust John, or intervene
John betrayed Harley (My choice was to trust him)
John refuses to give up the virus, which leads to a QTE section eventually of John blowing the explosive
John jumps off the bridge, officially changing him into the Joker (Which is what I will now be referring to him as)
Episode Complete