To style yourself a proper cultist, you must worship at least two gods, or an even number. These gods need to be as close to polar opposites as can be managed, so that they balance eachother out. How you decide to follow their commandments is up to you, whether you try to find some common ground in both, or alternate between the creeds somehow. The keyword is balance, for every evil there must be good, and vice versa. Only true maniacs would worship more than two though, as it is hard enough to balance dual deities.
For instance in a D&D setting, Pelor + Asmodeus, Erathis + Gruumsh, or Lolth + Corellon might be some interesting opposites to explore.
Because of the diverse nature of the cult, everyone is welcome, yet there isn't much that hold them together. Many proclaim themselves as prophets, or orators for the cult, resulting in smaller cells breaking off to pursue their particular brand of what is considered true dogma. And because of their worship of evil deities, they're no favorites among the authorities either, so like most cults, they keep their activities mostly hidden.
The trouble with balancing two or more opposing dogmas, and the moral choices that come with them, drives most cultists to insanity, or at the very least, keeps them balancing on the brink.
Most self styled cultists treat it as a quaint pastime, a way to entertain guests with a touch of the occult. And usually end up bored with the whole notion after a week or two. That leaves the fervent believers who fear the wrath of one or more gods, should they choose to cast off what they have brought on for themselves.
By its very nature, it is a chaotic cult, that usually lack any specific long term goals. But any one driven group are likely to try to stop the most unexpected thing, because they perceive it as a threat to their brand of dogma.
Their mark is a balanced scale(with different cells adding variations as they please), but because they usually lack both organization and funds, they have no real established dress code, other than the stereotypical robes, which are usually brown or any old bathrobe they could find. They have no real rites to speak off, other than telling eachother about their weeks and the hardships related to dogma they've had to face. If they are feeling traditional, they might chant for a few minutes, using an ancient word which meaning has now been lost; "lagom". Each group usually invents their own rites depending on how ambitious a leader they have.
Uses in Your Campaign
They can fill any niche, from comedy relief/touch of color to dangerous radicals. Easy to adapt to any pantheon.