Redirecting to another droplet from a secure droplet through subdomains
I have successfully created a droplet with Ubuntu 16.04 running my secure django app through Letsencypt. My droplet serves "example.com" and "www.example.com" and I can see the green lock on my firefox when pointing to this application.
I would like to create two subdomains (sub1.example.com and sub2.example.com) but on two other droplets. I would like to know the exact procedure if possible.
I understand that the first step is to "expand" the letsencrypt keys to include the two new domains. But what comes next? Please, can you be as much elaborate as possible? Please, keep in mind that the new droplets will have to be secure, too.