Before developing, clone the repository and run yarn from the root of the repo to install all dependencies.
From inside the project folder, run:
yarn start:lambda
This will open a local server running at http://localhost:9000 serving your Lambda functions, updating as you make changes in the src/lambda folder.
You can then access your functions directly at http://localhost:9000/{function_name}, but to access them with the app, you'll need to start the app dev server.
While the functions server is still running, open a new terminal tab and run:
yarn start
This will start the normal create-react-app dev server and open your app at http://localhost:3000.
Local in-app requests to the relative path /.netlify/functions/* will automatically be proxied to the local functions dev server.