How to get SSL certificate for homemade webserver

I've created my own webserver using Node.js ans is using it with my DNS from webhost routed to my raspberry pi at home. How do I prepare the webserver and all the files for getting an SSL certificate using Let'sencrypt?

I've not made any work prior to getting the SSL other than creating the server to run on localhost. Now it's running on a public ip while using my purchased DNS.

I haven't been able to find any litterature other than the off the shelf webservers like apache and such and I really want to run with my own.