Syncing Comcast.net email with new Yahoo account on Mac

Xfinity moved email accounts to yahoo, but when I go to yahoo for my mail I get a message that the accounts aren't syncing. I don't know how to fix this. I assume it has something to do w/imap and smtp, but don't want to mess this up.

[Re-Titled by Moderator]

Original Title: How do I sync my Comcast.net email account w/ the new yahoo account on my MAC