Install Node js from the site https://nodejs.org/en/ and download the 14.17.6 LTS version for Mac
Install yarn by opening your terminal and typing
npm install -g yarn
you can check if yarn downloaded correctly by typing
from your home directory.
You will access that folder in your terminal and type
run npm init @open-wc
You will create a hello-world boilerplate with a new project scaffold; a web component; all options checked for linting, testing, and demoing; does not use typescript; is named hello-world; and installed with yarn
npm run start
and you're all set!