how do I create an email alias
Comcast email has no provision for the use of aliases. You can create a secondary account with a different user ID, but that's not the same thing as an alias. However you could create a secondary and set it to auto-forward to the account/address you want to mask. You could do the same thing by setting up an email account with another email provider like Outlook.com or Yahoo or something, then set that to auto-forward to the Comcast account.