Detailed instructions on how to install SSL on a single mapped domain within subdirectory structure

I have a multisite network that has a subdirectory (not subdomain) structure. I'm trying to install SSL on one of the mapped domains. I've successfully used the wpmudev Domain Mapping plugin to map the domain, which is working fine.

I know how to use a single SSL for the entire network (which I have working properly), but am trying to get an SSL attached to a specific mapped domain, so as to the avoid the ugly certificate mismatch warning. Once I know how, I plan to repeat the process for all my mapped domains on the network.

Can you point me to any articles or write up some instructions on how I can accomplish this for a single mapped domain? I have a VPS with SNI and Lets Encrypt installed. cPanel, WHM, and SSH root access of course. Thanks!