very alpha right now
The blog post explaining this initial version; see updates relating to this.
See my itch.io creation for what this is talking about.
The code for this is at https://github.com/ordinalM/md-chapbook
Once you click "Create" you will be taken to a web page that looks like your Markdown, formatted for screen reading. Hit ctrl-P or cmd-P to print and it will appear in Pocketmod layout, ready for printing and folding. You should adjust your browser's margins, header, and footer settings to remove all of them, and also change the paper size to your preferred one. Feel free to save this resulting web page and use it however you want.