You're writing as a character in need of some help. The nice thing about writing in the first person is that your character might not be formally trained writer. If you've got grammatical errors, that's fine. Perhaps they're intentional! If you tend to write like an academic, write about an academic conflict. Easy peasy.
Take your time.
Letter-writing is a lost art for most of us. Hell, I wouldn't have been able to contact most of you if it weren't for the convenience of instant communication. Still, there is a craft that is inherent to the slower, deliberate pace of writing by hand. Take this into account as you write your letter. (That is, if your character is writing by hand at all.)
Personify the conflict.
Because it'll take a while for a letter to be answered, a flash flood happening RIGHT THIS SECOND isn't quite in the purview of the temple. You can still include natural disasters as part of your conflict though. Perhaps a saboteur destroyed a dam upstream? Is a water spirit angry at the villagers for some reason? Maybe the flood hasn't happened yet, but it can be prevented with the temple's intervention. The flood may simply be a natural disaster, but it has caused clashes over newly limited resources. In any case, make the conflict something that the players can interact and negotiate with directly. You can even write multiple letters ab out the same conflict from different perspectives. That'll add quite a bit of nuance to that session's play and beef up the book as well. ^_^
Bring up the problem, but not the solution.
The nice thing about the letters is that you're introducing unstable situations, but you don't have to worry about resolving anything. In a sense, the letters are the "dungeons" of Do. A series of precarious, sometimes outright dangerous events that the players must navigate before reaching their goal.
In other words, do not write a story. The story is what's going to happen in play. The letter is the prologue leading up to the Pilgrims' arrival.
Feel free to establish consequences.
Sometimes it's pretty obvious what will happen when national negotiations break down or when relief efforts are mishandled. But it's also perfectly fine if you, as the character writing the letter, introduce ultimatums, threats or "unfortunate circumstances that might result without immediate assistance." Let the p layers know the stakes they're dealing with.
Problems come in all sizes.
It's tempting to speak as an ambassador for a great nation on the verge of war, but all sorts of people have issues that need to be addressed. Troubles in the home can be as interesting as high-stakes diplomatic negotiation. Plus, a monk can only reach enlightenment when facing the full breadth of life's affairs.
Give the players some toys
Don't get bogged down in too many details, but feel free to introduce setpieces, props and scenery that is unique to your letter. As you write your letter, it might help to make a list of three or four little toys that the group can play with. For example:
Star Wars: Magic swords, spaceships, robots and a cruel Empire.
Lord of the Rings: Elves, a powerful ring, a reluctant king, and an all-seeing eye.
Avatar: The Last Airbender: Elementalists, hybrid animals, reincarnation and nature spirits.
Once you've written your list of toys, you can weave them into the letter itself. For example, if you've introduced magic swords, you can weave them into a heist caper or an epic quest. If you've got nature spirits, make them the letter-writer (or maybe even the antagonists).
Where's the Stash?
Add a quick note in the visual description about where the letter-writer hid this letter. This is a little nugget that can help players get a grip on where to begin the adventure.