Check out the project from github:
git clone https://github.com/bjrn/gatsby-with-netlify-cms.git
cd gatsby-with-netlify-cms
yarn
Start develop mode:
yarn develop
After startup, there's a local website running on localhost:8000.
The editor is located under http://localhost:8000/admin/.
In development mode, editing documents is file-based,
ie. saving a document will update the document on your harddrive, unlike editing on the built site, where document changes result in Pull Requests that can be merged via the publish button in the built-in workflow (if editorial_mode is enabled).
Local editing is great for making changes in many documents at once, or for migrating content from other sites.
see repo for a create-react-app version: https://github.com/talves/netlify-cms-react-example