As someone who first wrote her MC over a decade ago, has been adding to her stories for large chunks of that time, and spent over two years trying to nail down a comic concept, you may find that preparing for a year isn't enough time. But let me see if I can help answer your questions.
Full disclosure: I've only been posting for a little over six months, so I am not an expert, just someone who has thoughts about what has worked for me so far.
1. When should I post the first episode?
On a personal level, post when you're ready to. What 'ready' means for you might be different from what it means for me, but I like to find the sweet spot between 'I'm impatient and can't wait any longer' and 'I'm stalling'.
On a 'what's the best time to post' level... this is a little harder to answer. I will say that most of the comics I follow seem to update on a M/W/F schedule, or once a week (seems to be either Wednesday or Friday), and I personally update once a week on Saturdays, but there doesn't seem to be a 'magic' day that will somehow get you the most views. Maybe I'm wrong, but I haven't seen any real consensus to this.
2. Do I really need to create 10 or more episodes before posting the 1st one?
Do you NEED to? Technically, no. But is having a buffer of pre-done episodes really nice? Oh god yes. One of the best things you can do for yourself and your readers is post consistently. Someone who posts consistently is someone readers trust and come back to for content. Having pre-done episodes buys you time and grace in case life gets in the way of drawing for a while. Big project for school, no time to work on your comic right now? Thank goodness you have several weeks of episodes already done! Broke your hand, can't use it for a couple months? Wow I'm glad I made a bunch of episodes ahead of time! I can heal and not go on hiatus!
It also gives you time to figure out a few things, like "Can I really commit to drawing page after page after page for the long term?" or "Wait, maybe this scene would work better if I...?" BEFORE you're expected to post an update.
3. Do I need to write down everything that should happen, like a full on script or description of what should happen?
Again, do you NEED to? There are no hard rules that say you HAVE to. But knowing what's happening next DOES make drawing it easier. I wrote a rough script for the chapter I'm working on, complete with what happens in each scene, who says what, etc. It's not sacred, though-- I'm editing and cutting out and altering things as I go, because once you get from script to actually making pages, you figure out what works and what doesn't. But not having it nailed down first is more challenging.
The other advantage to scripting ahead is the ability to put in foreshadowing, or drop hints. This is a LOT easier to do if you know what happens next, and when and where those hints should be hidden.
4. PROMOTING! How do I gain people's attention about this project?
Thiiiiis is a place where I'm less experienced-- I don't have a huge following, and my subscriber count thus far is modest. I promote on the forum, in the promotion-specific threads (I avoid the sub-for-sub stuff though), and I cross-post on Webtoon, Tumblr, and DeviantArt. I also promote my comic on my Instagram, and I'm starting to see more activity there, so that looks hopeful. I don't have Twitter and I'd rather eat my socks than go back to Facebook, so those avenues aren't part of my promo lineup, though I know some people have luck with those.
Short version, though: There is no magic bullet to get people suddenly flooding your comic. Webcomics are a long game. Building readership is slow and (hopefully) steady, sometimes with little bursts, or losses. Be prepared to NOT be popular any time soon. We're talking months if not years. Your mileage may vary, but don't expect miracles.
5. Backgrounds are so difficult... help?
Ah, backgrounds. One of the most loathed parts of drawing for so, so many artists.
Honestly, one of the biggest things that I ever did to make backgrounds more interesting? Give them a 'personality'. Drawing a character's bedroom? Don't just make it a stereotypical bedroom with a bed and a closet and etc. Boring! Throw in something interesting or weird. Do they have a pizza-shaped pillow on their bed? Is there a bookshelf where you can put odd trophies, books with Easter Egg titles, or things that might be hints about the plot? Is their room a mess? WHAT is that mess? Papers? Clothes? Is there a fish on the floor that the characters never discuss or interact with? You gotta have some fun with backgrounds, or they get boring-- for the artist AND the reader.
On top of making them interesting... don't draw 'backgrounds'. By that I mean, it shouldn't look like the blue backdrop screen you sit in front of for school pictures-- a background isn't "behind" the characters-- it's all around them. They should be IN it, interacting with it. Sitting on beds, pulling out chairs, throwing crumpled up papers into the trashcan and missing. It's not a backdrop, it's a PLACE.
Another thing that helps with environments is... plan them out. If you're drawing a bedroom... where is everything? Make a quick map. Figure out your colors. It'll help things stay consistent. Knowing WHAT needs to go WHERE makes it easier-- you don't have to think as hard. I second the above advice: a 3D modeling program to design a room and use it as reference is very useful. Some people even build rooms/houses in the SIMS and use that!
Additional recommendation: If you've never done this before, and you're realizing how hard it is... start small. If you have an expansive, years-long universe full of stories, that's daunting and about 10x more work than you think it is. So try making a comic of a scene. A short, relatively contained moment between a small handful of characters that you're excited about. Actually finish it. And then go seek out a good, kind, thorough critique. Listen to the points, both good and bad. Evaluate, change what you need to change, and figure out where that leaves you, and what you want to do with that information when/if you move forward with a bigger project.
And make comic artist friends -- read and comment on the comics of people doing something similar to you, and reply to/interact with people who comment on your work! People love to interact with the artist/author, and will usually keep coming back if you're consistent with posting and interaction.