Personally, I find it helps to have a little more than just what's in Fronts: the cast of characters and their motivations is a good start, but I find it also helps a lot to have a map (if it's a dungeon crawl) and a list of some interesting treasures. Also some monsters with their moves, and an idea of what they're doing here, and probably a trap or two. You probably don't need full room descriptions like in a published module, unless you have trouble making that up on the fly -- just put in the monsters and treasures and traps when it seems appropriate. These things can be re-used from session to session, too, whatever the characters haven't encountered yet. The idea is to just to make sure that you don't draw a blank when the players are looking at you to say what they find.
It's easy to underestimate Grim Portents from the front, but I think they're really important. Or actually, if I'm in a hurry, for prep purposes I find it easier to write a list of Things that Might Happen without necessarily writing them properly in the form of Grim Portents. After you've had a session or two getting to know the NPCs, it might get easier to write each faction's plans in more detail. (Or maybe that's me getting more used to the form in my head.)
You say you're concerned about railroading, so the important thing is that you keep clear that it's Things that
Might Happen -- you can have totally contradictory items on the list, even without the PCs getting involved. Maybe Grundloch finishes his ritual first, or maybe the lizardmen overrun the caves, forcing him to get involved in the turf war before the scrying pool area is safe to work. You don't know which will happen until the PCs get involved -- maybe it's neither! But the ideas are there, so you can run with whatever happens.
If you're short on time, I'd recommend an iterative approach:
* Name a couple factions
* Describe the leader of each
* Write a monster description for the grunts of each
* Write a couple items of What They're Up To
* Sketch on the map where they live
* Write an interesting treasure item for each
* Write a trap
* Repeat the list and add a little more detail to each, or maybe a new faction, until you run out of time and the players arrive.
Check out The Shallow Sea on the DW website. If you like that style of first session prep, I have a readme file that goes with it, which explains the logic of the format and how to put one together. Let me know if you want the file.
I'd love to see that.