First you need to generate a CSR (Certificate Signing Request).
See for more details How To Generate An SSL Certificate Signing Request (CSR) In CPanel
Once the CSR is generated, you need to provide it to the Certificate Authority like Comodo, RapidSSL etc and they will provide you the SSL certificate and CA bundle in zip file.
Now Login to cPanel
# Click on Security >> SSL/TLS >> Install and Manage SSL for your site (HTTPS)
# Click on Manage SSL sites
# Select the domain name from the drop down.
# Paste the SSL certificate in the Certificate: (CRT) box and CA bundle in Certificate Authority Bundle: (CABUNDLE) box.
# If you click on Autofill by Certificate, private key will be fetched automatically.
Check the tick box Enable SNI for Mail Services and click on Install Certificate.
Please do contact us at [email protected] if you require any further assistance.