Have you checked out the quickstart guide? You need to fork manna_harbour/miryoku_zmk and use the workflow found in Actions > Build Inputs.
Personally, I prefer to take manna_harbour's excellent miryoku layout as a basis and write my own keymap, using the existing documentation as a guide. There's alot more trial and error but you'll have a better understanding of how the firmware works when you've got it working. I find the GitHub Actions-based workflow for building firmware overly complicated for my personal use case.