The HTTP Secure (HTTPS) protocol encyrpts data between a web server and the client web browser.
How To Secure Nginx with Let's Encrypt on Ubuntu 18.04 walks through how to configure your Nginx server with HTTPS using a free Let's Encrypt domain certificate.
Switching Your Site to HTTPS on a Shoestring Budget shows the steps for moving a GitHub Pages-based site or any site that can be behind Cloudflare's free tier to HTTPS.
The 6-Step "Happy Path" to HTTPS covers how to obtain a free SSL certificate, permanently redirect HTTP to HTTPS and fix insecure references to non-HTTPS resources.
HTTPS on Stack Overflow: the end of a long road is a wonderfully in-depth post on transitioning from HTTP to HTTPS, including redirecting all HTTP requests to HTTPS, for a very high trafficked website.
TLS stats from 1.6 billion connections to mozilla.org provides real-world data for which ciphersuites to use based on mozilla.org connections.
HTTPS in the real world is both an accesible read and gives the nitty gritty details beyond how the HTTPS handshake and transmission work, along the assumptions it breaks down about where the most likely attack vectors really come from.
How Let's Encrypt Works is a primer on how the free and now widely-used certificate service grants and revokes domain certificates.
Performing & Preventing SSL Stripping: A Plain-English Primer explains the KRACK Attack for stealing data despite an HTTPS connection and how your site needs to use HSTS to prevent the attack.
HTTPS adoption has reached the tipping point shows data about the growth of HTTPS and how most sites now serve more than half their traffic via secure connections.
Google's HTTPS transparency report card shows the growth of HTTPS connections across Google's properties. There is a clear upward trend but even some of Google's properties still are not 100% using HTTPS over unencrypted HTTP connections.
How https works is a fun cartoon illustration that demonstrates the basic concepts of a secure HTTP connection.