Hello and welcome to the web hosting hosters helpdesk!
To begin with you're client will require a dedicated IP for the site in question. They will of course, need to select an SSL certificate product if one is not provided by yourselves. You, or they, will need to generate a CSR within WHM ( or cPanel if you give them this functionality ). The CSR is then sent off to the Certificate issuer whom will provide you with the necessary bits and pieces for which you need to install ( or again your client can do this if you give them the functionality ). The process varies from cert to cert, so consult the documentation provided by the supplier of the certificate for the specifics. Hope that helps.