As the title says, I'm planning on begin running WotBS for 5e (probably in a couple of months), and I would appreciate any advice on what preparations are recommended for it, especially for the first adventure.
War of the Burning Sky is a rich and complex adventure path. It can be a rewarding experience to play with a bit of preparation.
Integrate the character's backstories into the adventure.
I would recommend that you encourage the players to use the Player's Guide to figure out where in the lands their character came from as well as what brought them to Gate Pass. Once they do that, you can look at which chapters contain good places for you to weave the characters' backstories into the adventure.
The following has what has been going on for my group.
[sblock] Trinja: A 14-year-old girl who was run out of her Sindairese village because her luck would cause accidents to happen to other people. She sees a dragon in a vision who chooses her to right a wrong. The player made her a Hexblade warlock with the Lucky feat. I described her use of lucky as time slowing down, allowing her to change her fate. The party revisited her village on the way to castle Korstull in adventure 6, and she will meet Time, her patron, in adventure 12. I also have added a bit of tug of war in that she inherited Takasi, but he warned her that she would not be able to re-summon him if he dies unless she becomes a paladin of the order of the Aqualine Cross. I am curious to see if she emerges as a Joan of Arc type of character.
Byron Cassanova: An up and coming blue tiefling cirqueliste who was exiled from the group after he had formed a forbidden relationship with a daughter of Khaegen Onammdamin and was betrayed by fellow cirqueliste Giorgio, which caused the Wayfarers to have to leave Ostalin and settle in in Seaquen. They encountered Onammdamin at the beginning of adventure 5 and ran into his girlfriend as the brainwashed second in command of Pilus later on. She died, killed be a fearful Three Weeping Ravens after they freed him from her grasp. Byron is not handling it well, but the player doesn't know that story is not finished yet.
Dro'marri Oldenforge: A dwarven monk in red overalls. And he jumps. He hails from a dwarven stronghold near the Alydi Gap in Dassen. When he was a child, a group of orcs led by Bowser defected from the nascent Ragesian army and became trusted soldiers in the dwarven clan led by his father Mario. This created grief for Dro'marri, which compounded when Mario died on a mission on the cliffs. Bowser ultimately married Mario's widow peach and led the clan. Dro'marri was a teenager who wanted to pursue herbalism rather than warfare and the forge, as Bowser insisted, and he ran away to find his own way in the world. When the heroes came to the Alydi gap, they were greeted by his Dro'marri's half-brother Luigi, and they got a chance to have dinner at the stronghold. Dro'marri found that Bowser had become a respected leader who had integrated the orc and dwarven way of life. Before he left, Bowser made sure Dro'marri took his father's old bag of earth.
D'buld Utch: An aged advisor to Khagan Onammdamin's father, D'buld was exiled when the son assassinated his father. He travels the world learning tactics and warfare. He acts as a father figure for Trinja.
His backstory has been mysterious for the rest of the group until adventure 7 when he executed one of the Droalesti assassins while others were trying to save her to get information. He explained that assassins are the worst destabilizing force in the world and should be destroyed whenever they opportunity arises. I think he might get some resolution with Onammdamin before the campaign is through.
Zed Tracow: A half-orc who grew up a slave with his orc father in Shahalesti, he escaped and settled in the militia of Gate Pass. What he is about to learn from some upcoming visions is that his father lied about his age whenever asked to hide that he escaped with the infant Zed from the Taranesti purge, where his mother died.
So there have been/definitely will be ties to various heroes' stories in adventures 3, 4, 5, 6, 7, 11, 12.
Balance the adventure path's constraints with the heroes' self determination.
Back in my session 0, I told the players that this was an adventure path. They would be given missions that they should accept and attempt to fulfill for the good of the story. That being said, they would have freedom within the mission to fulfill it how they saw fit. One place this worked better than I expected was in adventure 2. The fire forest only has a few paths the heroes can take, and I knew that some previous reviews described it as a railroad. When the heroes made it to the first crossroads in the fire forest, I nervously let them take whichever path they wanted and juggled the order of events to fit their choices. When we reviewed what went bad and well for the first third of the AP, the player's brought up the freedom to make choices and having their choices matter in adventure 2 as one of the highlights.
The heroes don't always have to win, but it helps. War of the Burning Sky is a modular adventure path. Most adventures happen in a different part of the lands. This means that most of the adventures can be "failed" by the heroes while the story still continues. Even seemingly disastrous failures can be integrated into the future adventures with small changes.
The AP has variable difficulty.
The AP has a difficulty adjustment built into every encounter. This is primarily to accommodate parties of various sizes, but it can also be used with groups that under perform or over perform in combat. Another thing the AP provides is nearby help that can fade into the background when it isn't needed. In the first adventure, Torrent is available, and later on Rantle can bail the party out if they are overwhelmed. In the end, Haddin and Cristen stay out of sight unless Haddin determines they need to intervene to have their best chance of surviving.