Step-by-step guide
- Ben Hollenbeck Convert can get you a certificate from a service like Digicert and Yamini Kannankutti (Unlicensed) can Convert the Certificate tos in .pfx format and format
- The Salesforce Administrator can then Install the SSL certificate as described below
https://help.salesforce.com/articleView?id=000339841&type=1&mode=1
Merge a complete certificate chain for custom HTTPS domains
https://help.salesforce.com/articleView?id=000333504&type=1&mode=1
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