SSL installation on my sub domain mapped to local server's IP Address

I have a domain name which runs perfectly on https" and I have access of hosting for this domain by Cpanel. From my domain I created a sub domain name and mapped (By A record setup) this subdomain with my local server’s IP address (Static IP) on which erpnext application is installed. Now I want to install SSL certificate on my subdomain by let’s encrypt CA (Free SSL certificate). I explored a lot but couldn’t install using certbot. I want a step by step instruction of SSL installation, please. Thanks in advance.