Seriously? The same way all of them start out. You take a few ambitious and violent idealogues who have no issue with creating a criminal & ideological enterprise, and they can sustain themselves through violence. Once you become a clear force to be reckoned with against populations you intentionally disarm, you can begin holding territory, demanding protection money, and bribing cops. You keep this pattern of behavior up until you seize multiple cities. Once you have a large enough base, you can advertise yourself among the rest of the jihadis with your suggest and start pulling in funding from Qatar, Iran, Saudi Arabia, or whichever religious sect you happen to work within.

They only put up fliers once they're long established. Normally recruiting (like any gang) typically starts with "join us or die". Once you join, you can get money and prestige. If you are effective in your violence, you'll get more money, including a kind of terrorist life insurance if you get killed in combat. That incentive even includes suicide bombers. Most of the time they are recruited either from middle-level educated radicals in academia, local poors, radical imams recruiting in the prisons for violent people, or radical imams recruiting from ideologically possessed foreign fighters.

This is pretty well established shit.