So I'm filling out the form, what do I put in the letter and description boxes?
(btw, I hope you don't mind me doing a public answer to this, but I thought others might have the same questions!)
Letter is completely optional. That would be if you had more details than you wanted to put into the description box and gives you the option of linking to an external Tumblr post, Google doc, or the like. You can just leave it blank.
In Description (which in retrospect I probably should've called something else), put in any details for what you want or don't want to help your creator. This is also optional, but highly recommended! If you click on the Requests Summary, you can see the kinds of things that other people have put there.
Usually the most important thing to put here is anything you really don't want that would ruin a gift for you, and you can also talk a little about what specific kinds of things you might like - e.g. do you like smut, do you definitely not want smut, do you like action and case fic, do you want to suggest some scenarios that you might enjoy in a gift fic? Another thing you can talk about here are some of the things that you enjoy in canon about the character or pairing you requested.
Like, as an example, if I had requested Peggy and Jack, I might write something like this: "I really enjoyed their banter and slowly developing friendship in the show (episode 1x05 is my favorite!), and I'd prefer to have them working together and getting along rather than being on opposite sides. Some things I would enjoy: hurt/comfort, working a case together, an AU with magic or shapeshifters (I love werewolves!), stuck in a hotel room with only one bed, bickering during a case gone wrong, or having to deal with a baby or kitten or other small cute creature. Please, no major character death or other pairings for Jack."
Or something like that! It can be long or short, but it does help to tell your creator some of the types of things you like. It's hard to create for someone with no guidance at all.
I hope this helps! And thank you for signing up! :D