If you haven't already, follow the instructions to fork the repo.
Then pull the code locally. You can get the git instructions after pressing the green button Code.
Once you have a copy of the code in your machine, you will need to start Storybook. To do so execute:
# Ensure the correct version of node.
# See https://github.com/nvm-sh/nvm#installing-and-updating to install nvm if necessary.
nvm install && nvm use
# Download all the project dependencies. You may need to install Yarn.
yarn install
# Add the following text using your editor.
echo 'PUBLIC_ASSETS_URL=/static' > .env.local
yarn compile
# Compiles Storybook in the current code base and shows the stories.
yarn storybook
This will open a browser page with Storybook in it. Storybook contains most of the documentation on how to continue, so let's get over there.