Look, just read up on the histories of Mormonism, Scientology, Unification Church, and others. Just take your pick.
What all of them have in common, however, is a charismatic founder who comes up with something which, on the surface, sounds completely fucking retarded.
"Oh, I found a book made of gold, and an angel told me how to read it. What, you want to actually SEE it? Well, it's buried. You can't. And you wouldn't be able to read it anyway."
"There's aliens, see, and they're inside us, and they're doing things."
Once you have a few adherents, give THEM some measure of power, something that grants them control over others, but not over you. As your cult starts to grow, come up with ways to punish anyone who tries to leave the cult. Some of them have quoted scripture that's super vague, some have just made shit up, and said "Oh if you leave the cult, there will be a mass shooting in a school." Then, of course, someone decides to leave the cult, and a few days later, lo and behold, a school shootin'! It's got to be something horrible, so they'll feel guilty, but common enough that it's bound to happen.
Also, you have to find ways to make yourself out to be the ultimate voice of whatever deity or being you're saying is behind the religion/cult. YOU are the only good voice, those who follow you are among the "chosen".