Most of my advice isn't helpful, but here goes.
Collaborative Posts: I like Googledocs. Everyone can access it, type concurrently, and there's a chat function. When I'm involved in a collaborative post, I encourage only one player to write the whole post from their character's perspective. Essentially, the non-writer(s) is/are just there to supply information, what their character/s do and say - and not to do any writing. Apart from direct speech, I encourage that collab's writer to rewrite all of the information they are given. This way, the whole post looks and feels like one solid post, just like any other. Otherwise, I find the flipflopping of perspective to be confusing, difficult to follow, and generally bad practice, since there's not really any difference between constant perspective shifts and a series of one-liners and you might as well do one-liners. It's worth saying that the system I've just outlined is by no means the standard, although I think it's had some popularity among players in the games I've run. It also has the bonus effect that you can blast through the exchange of information and plan the scene in one relatively short session ("so if my character says X, what does yours say?") and the writer can just take away the notes and write the damn thing in their own time.
Portraying Things Without Being Boring: My advice here would be, if it's not relevant, don't talk about it. Ships and buildings are pretty, and pretty is great, but don't derail actual content to just describe stuff for the sake of it. We call that 'fluff', as in, padding. I'm not saying that going into detail about these things is always irrelevant, but it easily can be. While I'm by no means the best writer in the universe, I find the best way to make my writing more interesting is to never use the same word more than once in a paragraph if I can at all help it, with the exception of 'the', 'a', and 'said'.
The Right Amount of Detail: How long is a piece of string? The right amount of detail is the right amount of detail that is needed. As above, if it's not relevant, don't be talking about it. Otherwise, it's fluff. If there's something to be got from detail, like how your character feels about how imposing a ship might be, or you want to describe something which will then come up in the near or distant future, go into as much detail as that needs. The least important thing to put detail into is your character's appearance and especially what they're wearing.
Getting into Advanced: I once played with somebody who was quite nervous about trying advanced as that was their first time. There was no marked difference between their writing or anybody else's that I picked up on. For me, I think the only thing that really separates Advanced from Casual is how much people are willing to invest in the games. Casual goes much more quickly while Advanced goes much more slowly, with predictable effects on post length, future-planning, etc. I've never ever seen anybody say to anybody else "you shouldn't be here" unless they were new to the forum altogether and were pitching something that clearly wasn't right for the section (ie: something that would fit better in Free), and, even then, it was constructive and not critical advice. Otherwise, GM's might be more inclined to request accurate spelling and grammar, but I've seen plenty of shonky SpaG in Advanced, and you seem to spell and use grammar confidently so it's not really worth your worrying about. Basically, go find something you want to join, and try it out. If you are really nervous, you could maybe say to the GM that you're open to advice and guidance.
There's no real reason to assume that Advanced is 'better' in any way than Casual in terms of writing quality: it's just for players looking for something maybe more in-depth than people in Casual might be looking for. Assuming it's about how well you write isn't strictly true. The longer posts that are typically expected will of course give your strong points more time to shine and maybe expose some weak points, but writing and reading longer posts would, in my opinion, give you a greater opportunity to improve your writing anyway.
Early Signs: I think that's number two on the list of things I've like to know, after the alchemical process that turns base metals into gold.
