It's not as difficult as it seems.
First of all, you need to find a group of enthusiastic people for the project who will serve as your creative team and, most likely, your cast. Of course, you can also make a series on your own.
Google Docs will be your best friend. Start by outlining the synopsis of what you want to do. From there, you can create the characters and begin developing a draft of the story and how you'll get from point A (beginning) to point B (end).
Don't get stuck on anything; you can change the story as you see fit.
Once you have your characters, basic lore, and other things you want to note down (for example, how Slenderman works in your series, what kind of things happen on camera versus off camera, slender sickness symptoms, relationships between the characters, anything else you think is important), you can start dividing the story into blocks.
I personally recommend that you develop your characters well. After all, you usually want them to seem like real people, so you need to understand how their minds work, and that will help you write the lore and blog posts.
As I was saying, divide the story into blocks like "Block I — Normalcy" "Block II —..." and so on. This will clarify which important points need to happen in each block, and it will help you develop them later and maintain order when writing the blog posts.
Do the same with your characters. Write a completely separate arc for each one. They're people, so it makes sense for them to have their own subplots within the whole mess. This helps you avoid getting confused and thinking, "WTF, wait, why didn't X help Z? What was they doing?" Instead, it makes you say, "Oh, X didn't help Z because they was kidnapped by a cult."
Once you have both the blocks and the character arcs, it will be much easier to write the blog posts. At this point, I think you're ready; it's time to start recording.
As for promoting your series, these days TikTok is definitely the app to do it on. You could also use Reddit, but personally, I feel like it takes away something I can't quite put my finger on, lol. Oh, and you can also post here on Tumblr in Slenderverse or Creepypasta communities, and obviously use the appropriate hashtags.
Even though these kinds of series obviously take time, and it's not uncommon for production to take months... or more. Fans can be a bit impatient sometimes. However, it's amazing to know that there are people genuinely excited to see what you're doing. I'd recommend that by the time you decide to promote, you've already uploaded a couple of videos so it keeps the engaged. Otherwise, another way is to announce you're working on your series at the same time you're working on it, and build emotional investment from the audience. This way, your viewers will feel more connected to what they're watching. You can even respond in character, just don't reveal any lore that no one is supposed to know yet, unless it's something like, "I've been really sick lately," y'know.
I think I had a few more things to say but my mind was working too fast and I forgot it while writing, but if you have doubts or I remember something, I will come back.
Ahhhhh thank youuu!!!! I appreciate the advice so much!!!