Everybody has a different process for doing everything.
Time to talk about mine.
Process, that is.
When prepping for running a game, I make brain stew.
I fill my head full of interesting things and then vomit that onto my games.
One of the things I've found to be true is that he who preps least, preps best. When it comes to me at least. I've tried in the past to write out detailed scenarios that would span months of gaming sessions and end in a satisfying way and, in general, what I find is that the more detailed and exacting your outline, the more doomed to failure it is.
The best sessions I've ever ran have been completely off the top of my head. It makes the game exciting for both your players and you and you're players so long as you can keep improvising and riffing off what they do. The game becomes a lot like free form jazz, except fucking awesome.
I do a minimum of prep work. 99% of my prep is making sure I have papers, pencils, and dice. But that last 1% is equally important. I take the bits about the setting, the bits about the system, and (if I have them already) the bits about my player's characters that I dig and just think about them. Usually in the shower or while pooping. Sometimes I just lay in my bed let all those elements stew for a while.
Once you've got a good pile o' shite in your head, it's time to start work on the "Gel de Groupe" which is only about half as gross as it sounds.
Why the hell would the player characters ever hang out together? Very rarely players will make characters who naturally fit well together, but most times, they create a motley assortment of undesirable and anti-social scumbags. How do you get the one-eyed half-jackal necromancer and the Priest of the God Who Punishes the Naughty work together with cannibal man who also a secret homosexual?
The short answer is you don't.
In my games, that responsibility (if there is one) falls squarely on the players. The most I do is give them a nice nudge. I say it's storming and have some NPCs recommend the same tavern to all of them, or have only one bar open that night or they all get arrested and share a cell. For the most part, I just demand that all the characters start off in the same city. That's usually enough.
Once you've got 'em all grouped up, make something happen that they all can find a reason to care about or at least be paid to care about.
Bandits attack. The well is poisoned by goblins. Plague sets in. Rumors of fantastic unguarded wealth. Just whatever. Anything to get the ball rolling. Everything else can develop organically from there.
What is important is to get a head full of ideas, all swimming round yer brainbox in a thick stew. Then ladle it out to your players and add hot sauce as needed.