How to start a cult?

gather a bunch of folks who think and feel the same as you about certain issues, subjects.. whatnot? make claims that will stick with them then.. well get registered in the city as a parishioner or whatnot for your own church and have them so to speak "come aboard" and if you get enough followers and are in your location LONG enough and are proven legit you can petition for tax free status!
dun-dun-dun.
but you'd need support and sponsors... you'd have to ask about that one with the 'faith' sorted in your region to learn more what you can do to open up a new 'base' and become associated.

you can probably find others who already done this in your county? state? ONLINE all you gotta do is just google this crazy organized faith crap and see who pops up and where.

Be or find an exceptionally charismatic individual, then have him preach some kind of nonsense. Rinse and repeat. That's all that's necessary. Humans are notoriously weak to charisma.

Spooky stories would suggest making a hideout in the sewer, then finding some ancient god to worship, buy a nice black robe, and then leave vague clues and cryptic messages around for potentially unstable people with enough brains to solve them to find and follow through on.

First, gain an audience

Charisma, excellent communication skills, a strong background in psychology, and an idea that plays on the vanity and fear of people in general.

Become rich, promise wealth and power unto others by giving them some, make them donate to something "greater", take the money and become even more rich.