Storywise, it just depends on locale. It's DEAD SIMPLE to get them started if they are in the right place. Basically, the adventure starts with them wandering along, seeing the town of Greenest and bad stuff happening, and rushing to help. So as long as they see the town, you're in, and everything rises organically from the first sighting.
If they aren't in the right place, you just need to give them a reason to head down there. One of the suggested bonds in the back of the adventure is that they know a character who's integral to ep 2 and they come looking for him. You could easily fit that into a character's backstory and have him send a letter to the PCs asking for backup in his investigation. Or just come up with any old excuse to be down there. One of my players has this big thing in his backstory where he has to kill "The Great Bear" for his tribe. I started the adventure with them coming back from chasing down a great bear rumor.
Mechanics-wise: This one doesn't seem that hard either. A first level party will get their butts handed to them if they don't try to avoid most of the encounters in the first episode. If your party is bolder with their higher level confidence, you might not need to alter the adventure too much, as hordes of bad guys are bad for anyone in 5e. If that's not enough, adding about 50% more bad guys to any scene will probably do the rest. There's a boss-style fight that is already handicapped for them that you'll need to adjust the handicap for. There's another at the end that I'd just play as is, because the adventure already is written to deal with what happens if they do better than expected.
The second ep has almost no fighting if they do it right. By the third, they are supposed to be 3rd level, so you might just let it ride.
I'd just tell them straight up that you're going to be a little stingy on the xp for the first few eps to sync up better. My players are all very cool about stuff like that as long as I'm up front about it. I did it with Kingmaker in Pathfinder recently, because I wanted to send them through Keep on the Borderlands as a prologue. For instance you could give them almost no xp on the second episode and I doubt they'd even think it's weird. Without doing the math, if you gave them a token xp award for the first episode and nothing but monster xp (no goal oriented xp) from there on until they match up with the adventure, I suspect you'd be on parity with the adventure difficulty by the end of the first book. You might need to fudge that down a bit more, but if your players know what's going on, they will probably be cool with it.