Run yarn to install everything. You are now ready.
Add your new screenshot settings into an appropriate key inside screenshots.js.
There's a test.js file, that hopefully is in sync with index.js regarding functionality. Set the variable at the top to control which screenshot to make.
Running node test.js should make a screenshot on your local machine the same way it will happen once you call the URL. Use this to develop a new screenshot and get the pixels/offsets just right.
Just push to master, the code will be live about 60 seconds after that.