Yeah, pretty much what everyone else has said.
There was a campaign I tried starting years ago and dived in for the big overarching campaign with mutiple levels of conspiracies and backstabbings and all that. 20 minutes in one of the players calls the opposing team with info they got...unraveled it all. A week's worth of prep and flow charts and stat blocks wasn't needed as through basic logic and reasoning the players cut through it all, hehe.
So I try to start small, mostly to see what i'm dealing with in both characters and players. Figure out how to include the non-combat types in combat, the combat types in non-combat, and so on. It never works as smoothly as that last sentence makes it seem, but that's what I go for. Maybe add some behind the scenes stuff for myself, but mostly just "here's a simple run...go." And Ganggreenkhan's last post reads like a perfect way to start things to me.
Then, as I figure out the PCs, their backgrounds, and how those backgrounds manifest in-game, then I start working on the layered conspiracy, whirlwind missions of chaos and confusion, drek-hitting-fan stuff.
And it helps having two GMs, at least for me, as our other GM takes the team to Chicago, running season 5 missions that gives me a few weeks to prep for the next non-canon run i'm planning. On that same note, running published missions/adventures also gives a buffer to your plans as GM while letting you see how the PCS deal with things.
As for mission ideas, my group's current campaign has gone with:
Missing person search and "rescue"
Black market goods deliveries/protection
Season 5, mission 1
Splintered State
Season 5, mission 2 (just started)
Googling "Shadowrun Mission Ideas" came up with some stuff, including a pdf, that looks to me edition irrelevant of plot ideas at savevsdm.com (not affiliated with them).