In case this hasn't been apparent yet, I'm a dark fantasy writer. So expect the 'bad guy' point of view in this answer.
Your starting point: a leader.
No rebellion, no revolution, no nothing ever starts without a 'visionary'. Someone with a silver tongue, that knows how to sway people -- the masses, preferably. This person will know the hardships of the people they want fighting alongside them. They'd need a compelling story to really stoke the fires of the people -- though not necessarily a verifiable one.
Example. Amon, from Legend of Korra. Or frankly, Trump. Bad examples, with arguably dark intentions, but people that speak to the paranoia of the people all the same.
Your rallying point: a common goal.
This part comes in two parts, true to history.
- First is the surface goal. This is what you tell the masses, what you allow everyone to hear and talk about. It needs to speak to the masses. So if they are oppressed? Freedom therefrom. If they are hungry? Food for them and their families. If they are poor? Equality and fairness, so that everyone can reap the rewards of their labours.
For Amon, it was freedom from benders. He played on the fears of those who couldn't bend, showed them what a renegade bender could do to people that couldn't bend -- and they are oh so arrogant, too!
For Trump, it's far more insidious. He plays on racial bias, the desire for "a great, Christian nation", and so many other things.
- The hidden goal. This doesn't always have to be bad, it could be simple lust for power. Or it could be something noble -- i.e. true freedom from oppression. But every movement has one. Even if it's as mundane as no longer wanting to work the fields anymore.
Your lieutenants: inner circle.
No matter how suave the figurehead, they need capable lieutenants to get the work done. After all, how many leaders fight a war on their own? Whether a battle with guns an spells, or a battle of wits in their palace among the noble you need to overthrow. You need a solid team backing you to get you in and out in one piece.
These are the people that need to balance your leader. If the leader is "a lover not a fighter", then the lieutenants need to be the best warriors available. If the leader is "the strong, silent type", he best be relegated to the housekarl and let the Thane take over (Skyrim reference, in case someone did't get it).
How to do it: the plan.
Start off small. If you are setting this in a city, start by recruiting in some neighbourhoods. Be sure to start off with the rambunctios ones. The idealistic youth is usually a good first target, though typically harder to control over long periods.
Gather a following. It doesn't matter how small, even if it's just your lieutenants at first. Then you start with the biggest issue:
Funding. No rebellion/revolution can go unfunded. So you start off by acquiring the funds you need. Whether this the 'Robin Hood route' (rob the rich to feed the poor), the Salvation Army route (all donations are welcome, whether monetary, time, or materials -- we'll put it to good use) or lobbying like you're gearing up for an election. You need someone you can trust to handle this (it doesn't end well, if blackmarket weaponsdealers don't get paid as promised).
Food. No one fights on an empty stomach. So make sure you keep yourself and your people (well) fed. And if they ever so much as catch wind that following you leads to a worse life than they currently lead, you'd best have one hell of a brainwashing scheme in play to keep them loyal to the cause.
Arms. No war is fought without both pen and sword, you'll need both close at hand.
Get these three things, and more followers will follow. If only because you feed them, they will follow.
Size really does matter: Scaling up!
Once you have the basics (every step before this one), then you need to start getting strategic. However, this is the step that is most world/plot dependent. So a few plausible routes:
- Divide and conquer.
A house divided unto itself cannot stand. So create that divide in the enemy. Find out every possible division point amongst them, and drive a wedge in there. Stir up old rivalries, play on racial divides (but make it look like they are the ones doing all the dividing, you're the good guy after all!).
- The monster that goes bump in the night.
Nothing freaks people out more than the Eldrith. The monster you cannot see, the monster you cannot comprehend. So don't give them an enemy, or a note, or a sign that it's your group doing any of it. Let their imaginations run wild, and make sure you keep an ear to the ground to see what that is turning into.
But to make this effective, you need to make your hits big. Assassinations with no sign as to what happened. Stores getting robbed, but no one sees anything. Start small, let the panic simmer a little. Then hit bigger and bigger targets each time.
More than that, if there are other groups that are known for causing trouble, or simply groups you want to fall and hard: set them up. Make it look like it was them, let the autorities take them down, hard -- preferably violently. This weakens them, while it keeps you out of range.
The endgame? That depends on your plot. Maybe it doesn't have one. Maybe, like the Joker from DC, the anarchy is the endgame.