Here's one way to draft an interactive story with Twine or Ren'Py: Just write it straight through. Write a linear story. Then, go back and find some interesting bits, interesting choices, in that linear story. Imagine a different way things could have gone. Add a branch for that. Keep doing this until you're happy with the amount of interactivity/branches. Maybe clean up the script at that point if it seems worth doing.
If you're using Twine I might also want to look for places where it feels natural to break the text. This seems less necessary for Ren'Py. With Ren'Py the player sees text one line of dialog at a time anyway. There are no player-observable "nodes." But with Twine, the player views text as continuous web pages. Generally in Twine each page corresponds to a node — so if you only have one node, the player just sees one long page of text. That is a different experience from clicking between nodes of text. Short pages and lots of clicking seems more typical for Twine games.
(is that a good thing?)
(With Ren'Py I suspect there's a similar question about splitting files, although I'm not sure if you ever split files with Ren'Py. It feels like that ought to be a possibility, anyway.)
This seems like a good approach coming from writing prose. Just write prose "like normal" and then come back and "write prose" some more. It's almost like you're writing an alternate universe fanfic for your story. Or it's like you're doing a second draft where you replace an entire scene or sequence.
I probably would miss out on some subleties or possible IF game tropes/elements coming from prose with this approach. That seems fine. Getting fancy can come later.
This could be an interesting way to edit a prose story: Pretend it's an interactive story and consider the possible branches along the way.