If you want to use Apache instead of Nginx as a reverse proxy, here's a sample config that will get you started.
Make sure to change the highlighted values to match your own domain and the port that Postleaf is running on.
This post will show you how to install Postleaf on a DigitalOcean droplet and configure Nginx as a reverse proxy. You'll also learn how to use pm2 to keep the app running, and optionally install a free SSL certificate from Let's Encrypt.
To complete this tutorial, you'll need a DigitalOcean account and an SMTP provider for sending transactional email.