Xfinity uses at least 2-3 different software packages, and many brands of boxes. Programming Favs will be different depending on the Xfinity box you have. Since we do not know that in advance, some experimentation is required.
Some Xfinity boxes require an OK signal when programming channels manually for Flipper's "Favorites" function. Others do not.
Look on the back of Flipper to find your model number (V followed by a number, V7, v8, V9, etc). Just try two Favs to test the format.
1) In Flipper V9 and above, the OK signal is in Flipper's LC button. So the format will be: 123-LC-ADD; 456-LC-ADD
2) In Flipper V8 and below, the OK signal is in Flipper's Mute button. so the format is: 123-Mute-ADD; 456-Mute-ADD; etc.
If your box does not require the OK signal, then just set Flipper as normal: 123-ADD; 46-ADD; etc.
As always, you can send us a note. Our number is always in the footer.