image courtesy of @banjofox
Create a virtualenv (e.g. virtualenv -p python3 venv), activate it (e.g. . venv/bin/activate), and then pip install -r requirements.txt. (Make sure it's a Python3 environment; some of our dependencies require it.)
Create an account with MongoDB's cloud service; it's free for a very small cluster. Set up an admin user with a separate password, create an IP whitelist and connect to the database cluster to make sure it's working. I use a blanket whitelist, and Atlas complains at me every time. (Install mongodb to your machine, with Homebrew, for example.)
Docs:
Update config.py wherever the information looks inadequate (e.g. anything that says .rowan.website should have your own domain).
Create a .env file (or otherwise configure environment variables), based off of the included .env.example. This is conveniently loaded in by python-dotenv.
For local development, you probably want to leave SSLify disabled (STRICT_HTTPS in config.py), to access the server over HTTP rather than HTTPS. The STRICT_HEADERS config variable is also disabled by default, so that you don't have to worry about content headers to debug the API server.
$ . venv/bin/activate
$ gunicorn run:app