CloudFlare Proxy Addon
This addon sets up CloudFlare IPs as trusted proxy IPs. In doing so it resolves the session invalidation you'll see if using CloudFlare
Composer Based concrete5
If you're using composer based c5, all you have to do is require the package and install it.
$ composer require concrete5/cloudflare_proxy $ ./vendor/bin/concrete5 c5:package-install cloudflare_proxy
Download a zip of this repository and extract it in your website's package directory.