What depth of information do you want ?
At the top level, simply use a client like certbot and it will do it all for you.
At the detailed, programming, level it's defined in the AMCE protocol
If you want specific instructions I'd suggest starting a new topic in help, and provide as much information as you can ( fill in the template form for domain name, host, operating system, if you are on a shared or dedicated server, if you use a control panel etc ) and we can provide specific instructions. Without that information it's difficult to be specific to your requirements.