Writing for the purpose of role playing can be difficult, before coming here a person should at least be somewhat familiar with the act (art) of creative writing. If one is not terribly familiar with creative writing, I suggest reading a few writing guides.
There are also hundreds of books on the subject. For our purposes we will assume that the players have some idea of what creative writing is and how to do it (even if they don't think they do it well at all).
We use a 3rd Person narrative. That means generally speaking one should write as if they are narrating someone else's events and refrain from using personal pronouns like I or me in the narrative text. This allows us to do a few things, for one, a consistent format makes it easier for everyone to read, and secondly it allows more variations as multiple points of view can be expressed easily. Rather than saying "I thought about picking the flower." and have no one know who "I" is referring to, it is better to say, "Joe thought about picking the flower." Now everyone knows who is thinking about picking a flower.
Text and Context
In any chunk of text there should be two primary elements when something is being said. There should be text and context for the text. That is to say, when you're reading a book you don't usually just see characters blurt things out without something about what they are thinking, doing, feeling, as they are speaking. So when typing, on average, a good role player with have some way to differentiate the two parts. This is sometimes called emoting.
Lance was thinking about heading back to his room, it had been a long day today but he didn't want to end the conversation too early, "Oh, no I appreciate the thought, I'd love to." he responded, the smile hiding how tired he was.
That is in opposition to:
Oh no I appreciate the thought, I'd love to.
See how much information is left out? With the first one we have a context, he's tired. We can imagine that his eyes have a bit of a tired look, we can think perhaps his attention or body language shows he's a bit tired and wanting to head back. The smile might appear forced. This gives the other players so much more to work with in coming up witht heir own responses.
Ways to Differentiate
We are not very strict, so any player is going to see a mix of ways to separate what is being said and the context. A lot of people use italics.
Lance was thinking about heading back to his room, it had been a long day today but he didn't want to end the conversation too early Oh, no I appreciate the thought, I'd love to. he responded, the smile hiding how tired he was.
It is a little bit clunky, but it does in fact work. Other methods such as bold text are also used, but less frequently.
What we'd like to stay away from is the multi-line approach:
Lance was thinking about heading back to his room, it had been a long day today but he didn't want to end the conversation too early Oh, no I appreciate the thought, I'd love to he responded, the smile hiding how tired he was.
Where each lines is on its own, that can get confusing and ping people multiple times for one action.
Third person text makes for the easiest read. However, it is not the only method which makes it clear who is being played, what they are saying and doing.
Pure 3rd Person
Writing in the 3rd person also facilitates writing for multiple characters at the same time.
Lance walks into the bar and slides into his seat, Finny comes over and asks him what he wants. "Oh, just a beer today Finny." Lance says with a smile, Finny responds, "You got it boss." and goes to get him a cold beer.
In this example, we know who is saying what and to whom. It should be noted in this case that both Finny and Lance would be being played by the same person, one should not automate someone else's character by posting actions for their character without permission.
Multi-Line 3rd Person
Another example is playing two characters separately:
Finny smiles at Alouette and slides over a plate of fresh tacos, "Enjoy Captain." Lance nods at Scott, "That's a good point, I'm not sure the difference in how it works."
Now we have two characters being played by the same person, but both of them are in separate conversations, by specifying the name of who is saying what to who, it is clear to everyone who is talking and who they are talking to.
When doing multiple characters sometimes people would prefer a screenplay or script style of writing, this is perfectly acceptable.
Finny: Smiles at Alouette and slides over a plate of fresh tacos Enjoy Captain. Lance: Nods at Scott That is a good point, I'm not sure the difference in how it works.
This style makes it clear who is talking, what they are doing, etc... It can be easier for a lot of players to do as well.