Make an existing folder a LongForm project

I’m butting my head up against a wall trying to make an existing folder into a LongForm project. I have a folder immediately beneath the root of my vault with a number of Markdown files in it. I’d like to make these scenes in Longform, but I can’t create a new Longform project from the root of the vault. I don’t want to create a folder then create another folder beneath that and move all my files from the existing vault to the new one if I can help it, since the existing folder is part of a larger Git repository and I don’t want to lose my history…

The Longform docs seem to promise I can create a new index file without going through the default project creation process, but I can’t find anything that does this. Am I just missing something?