Generally speaking the rules are that you shouldn't be rude, and just act normal. Other than that, not much.
When it comes to joining existing games, most quests are ready to accept a new player at any time. This has been a common attitude since the very first iteration of Maidens of the Kaleidoscope.
Just make sure that you catch yourself up on what's going on in the quest, other players likely won't do that for you. Thankfully, it's common, good etiquette for the GM to post a summary of what happened in the last thread of the quest. In the case of the quest's first thread, the first post will usually have a rundown of the character you're playing as and the overall situation.
When it comes to
making quests, it gets a bit more complicated since you're literally a GM.
I recommend that even if you don't plan anything in a quest, you write a draft of what the first post will be, since that's
the first post people will see. It will likely set the tone for the entire rest of the thread, so you better make sure the first impression will at least make some stick around for the first few posts.
When it comes to multi-player roleplay threads... Those don't really exist here at the moment. They were a rarity in the past but they're nearly extinct now. I'm planning on making one in the future since VanillaCake asked about Touhou TTRPGs and that made my brain start going wild. It's a relatively unexplored area here but on previous precedent (The now cancelled
Roll to Dodge: The Return Of The Crossovers) I can say that you should put the Rules for the Roleplay in the first post along with a template or example character sheet for the Players to fill out if necessary.
If everything I just said sounds idiotic or you just need a bit more detail, you can look at
this thread from the previous iteration of MotK, containing some tips for writing a quest and accommodating new players.
