You just need to install the plugin on the website, login with your Cloudflare account and then activate the service via a button.
You must do this on each site you intend to activate the service on and the $5 per month is for one domain. If you have a WordPress Multisite you won’t have problems, but if you have multiple domains it will be 5$ each.
I was on the same quest as you and this was the best solution and the one that cost me time, money and headache, believe me.
