Most of the design of the writing app is going to be pretty simple, I think, but there's one thing that's a little counterintuitive.

When you write in Horse Drawing Tycoon 2, you're using a markup language.

That language is called "Marquee", and it is explained in much more detail here:

https://marquee.cube-drone.com/

Marquee is a lot inspired by Markdown, but it also includes a lot of stupid horseshit.