conda install -c conda-forge mystmd
conda install -c conda-forge "nodejs>=20,<21"
npx mystmd init # first time
npx mystmd start # after that
Install mystmd to be able to render the reader.
I also had to install Node.js.
Then render the reader using either npx mystmd init for the first time or npx mystmd start any time after that. It will generate a local web browser thingy (often localhost:3000 or similar) in which you can view the reader in real time.