Just wanted to share something I had been thinking about when it comes to starting off roleplay/writing letters, since I am honestly a little bit stumped when it comes to mine. Obviously new introductions are easier (you just make the introduction as you would) but letters that would theoretically, for example, be one in what has likely been a chain of exchanges between, say, family members or long time friends is a little harder.
Anyways, my suggestion definitely isn't anything convoluted or profound, but I thought it might help anyways. Just arranging with people your character will have been in regular correspondence with who will be the first to send a letter in play, and the person who is NOT sending the first IP letter (ie, the character that would have in theory sent the last letter) can give a general synopsis on what that letter might have entailed (general topics, tone, maybe a single line or two just to get the feeling across, especially since it should be easier to come up with a few bullet points than an entire goddamn letter), so that the first in play letter can be formulated as a "response" right from the get go (as opposed to having to come up with a "response" to something that never actually existed).
Anyways, just thought the suggestion might help anyone who is also a bit stuck with figuring out where to start! Feel free to add any other ideas/tips you might have on getting started.