Step 1. Fork this github.com/fletcher/peg-multima … ex-support
Step 2. Go to your fork and grab URL for your fork.
like mine is this --> git@github.com:beacloudgenius/peg-multimarkdown-latex-support.git
Step 3: Run these in terminal
mkdir -p ~/Library/texmf/tex/latex
cd ~/Library/texmf/tex/latex
git clone git@github.com:beacloudgenius/peg-multimarkdown-latex-support.git ./mmd
Edit the contents of ~/Library/texmf/tex/latex/mmd as and when needed and optionally push your changes back to your github.
Now Scrivener will automatically pick up your customizations.