Such a wonderful question! Time to just vomit out thoughts!
Mindset/Setting up the RP:
Well, usually, it all starts with a really shallow idea. Something I like, too. My modus operandi way of thinking, for everything in relation to that, is to ask myself 'Would I like being a player in this, too?'. So, as I said, it starts with an idea I have. To give you two recent examples:
I'm in the mood for something with magical girls!
Reach of Titan looks pretty cool, I wanted to make something fighting big monsters in a grim-dark setting with some game-y systems for a while now, maybe I can adapt it?
After that, nothing. Well, not exactly, but mostly! For a while. You see, there are a bazillion players and GMs out there who suffer from the shiny-effect! I'm sure there is some stupid fancy/technical term nobody cares about for, but with shiny-effect I pretty much mean what I said, duh. You go 'Oh, shiny!' for a few hours, maybe days, this wave of enthusiasm carries you a bit longer, BUT(!), ultimately, it was just a mood. So you end up dropping the RP as either Player or GM and end up on everybody's shit-list for being unreliable if you pull that too often.
So, instead, I spend a few weeks not doing much. I do think about the RP, but not in a 'super dedicated' manner. Essentially some rough world-building(or rather, plot-building) while taking a shower, prepping simple food, or out for a walk to some other place and so on. It's certainly not a process of planning out the entire plot or anything, because if I did that I could just write a book instead.
It's more like I try to answer some 'core' questions for myself to make a believable (animuh) RP. Take a classic example: Darkdeath Evilman wants to destroy the World!
Why does he want to destroy the world?
Who helps him?
What is potential motivation for the 'heroes'? How did they all find together?
I don't like white vs dark plots but much rather prefer grey vs grey, how could I achieve that?
Any cool twists I can come up with and incorporate?
Stuff like that. So, after a few weeks passed, and I am still motivated for 'this RP', I start writing up the Interest Check and fill in more holes as they come up. By that time the plot consists of a setting it plays in, a reason the player chars potentially all venture together, and a common goal. There's obviously more fleshed out like key NPCs, key places, and so on, but not much beyond.
Why? Well, my opinion is that part of the joy of roleplaying comes from interacting with the players as a GM and vice-versa in an unexpected manner. To challenge your players to take the initiative, try out stuff, see where they take it. Maybe they side with Darkdeath Evilman just because? I can only put emphasis on roleplaying is roleplaying, not writing a book. If you plan out your entire adventure on a single path your players either feel like it doesn't matter what they do and/or the GM gets way too annoyed for the players being 'too noisy'. Even if you plan out different routes or something, your players will still end up doing something unexpected.
It's more-or-less all just a big improv play where both sides surprise each other (hopefully) in an enjoying manner.
Players and You:
That's the big one. To railroad or not to railroad. To say 'You're inside this village, do whatever you like' or be a guiding hand? Maybe some GMs have other people where they can be trusting that giving them too much freedom is okay. But personally I've experienced that your players always say 'I want muh freedom!' but if you give em too much, they might lost interest because they either can't or don't do much with it.
Usually, how I handle it, is to give multiple choices. Some choices could even lead to the same destination. For example, rather than saying "NPC leads you to the Tavern." or "Go gather information in the village. kthx I'll see you next GM post" I might say "Gather information in the village." followed by points of potential interests to gather said information. Again, each road could even lead to the same goal. Your players don't need to know that, they'll feel good regardless. Amongst these players, you still might have someone who asks you "Hey, can I do this instead?", or maybe they just do it. That's great! You can work with that they've given/surprised you.
Then there's pacing. I have been in RPs where you have 24-hour posting-cycles for 1 to 2 weeks, followed by a slower pace, to RPs with one post a week. It really depends on how the players mix well with each other. Someone who posts relatively fast might lose interest if there is a post only every other week because another player might only post once every two weeks. Then you could give (generous) deadlines. To which some players post at the last day, potentially making it feel more like a chore than enjoying themseleves I'm guilty of that <3. It's a delicate manner and what I found out, works best for me, is to be in constant communication.
My current, running, group RP doesn't have a deadline but it's roughly on one post per week. I communicate a lot, tell them if it's too slow for them they should tell me! Keep emphasis on how communication is important. And so on. I can do that because...
...player numbers! I could never ever make an RP with 10+ people work. My preferred number is four, maybe six. Essentially each player deserves your attention the same. See how you can incooperate what they have given you in their sheet, or the on-going RP, into the RP. Make them feel engaged. I can do that for 4, I can do that for 6, eeehh. But more people? Urgh. That said, I do accept more people since there are always "oh, shiny!"'s amongst them.
Learning:
Honestly, mistakes seems to be in every RP. I feel like in every RP I ran I learned something. My Fortissimo RP I ran here a few years back? I gave too much freedom. My Etrian RP I ran at another place? I was too harsh with a boss. My current magical RP I'm having at another place? Trying to adapt a very succesful system from another forum RP page was a mistake since my players aren't used to it. It's not as magical girl trope-y as I wanted. Their first location to fight 'evil' being a gorey-flesh-dungeon-place might not have been the bestest idea. And so on.
The thing is, you can't change others, only yourself. So never try to find the fault/blame in your players. It's a constant journey. Although I have already given the biggest piece of advice in the other things I mentioned here.
Worldbuilding:
I think you didn't really ask about world-building in your question but I think it's important to mention a bit about it anyway! Hence it being last.
Okay, so world-building. Actually pretty simple: No reason to re-invent the wheel. You don't have to be unique. With all the fiction that exists nowadays you hardly can be unique. Most likely whatever you do, someone somewhere out there had already done it. You don't have to be different just for the sake of being different. It doesn't matter if your roleplay plays on a 'standard medieval fantasy-world'. If you tell in your OOC it's a 'standard medieval fantasy world' and give a few sentences to give a more concrete picture, potential important places, races, potential racism and so on, that can be enough.
My point is, less can be more. What's important is what the players (make you) experience and vice-versa. And how much 'world-building' is really needed? You can have the Kingdom of Uwupls inside the world of Nekosway with its own currency, language, marriage(Polygame? Age? Consent?), culture, date-time-system, and so on. Or you just don't. It's a question of building for the sake of building versus what extra enjoyment does it bring? Because roleplaying is a hobby, and you should enjoy hobbies.
Aaaannnddd that's pretty much it. I don't think I would read this wall of text myself but whatever. Just blerping out thoughts. I can only put emphasis again on how communication is, at least for me, very important!
