There are many boxes in the field. They don't all work the same. Even the same box can work differently, depending on the firmware used by the specific cable provider. And even the same cable provider can use different firmware, depending on the location and equipment.
So unfortunately channel entry format requires some experimentation.
First we will provide the usual format, then provide alternatives as well as specific format for some but not all Xfinity boxes.
Look on the back of Flipper to find your model number (on back, V followed by a number, V7, V8, V9, etc). Always program two Favs to test the format.
First, Flipper has three possible channel entry formats:
1) Just digits: 123-ADD; 456-ADD. (a) Sometimes using 0's as placeholders can make channel changing faster for lower digit channels: 0001-ADD; 0015-ADD; 0123-ADD; etc.
2) Use of the OK signal: (a) 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;
(b) 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.
3) Use of the Enter signal: (a) In Flipper V9 and above, that signal is in the Mute button: 123-Mute-ADD; 456-Mute-ADD; etc. Flipper V8 and below has a combined OK / Enter signal, in the Mute button: 123-Mute-ADD; etc.
NOTE: some boxes the OK / Enter can cause a Menu to appear. In other boxes, it will only appear if the OK/Enter is used in 4 digit channels. If this happens, then do not use the OK/Enter signal, or only use it in 3 digit or below channels.
To Program:
1) Press and hold STB and ADD at same time till light solid;
2) Enter channels: 123-ADD; or 123-Mute-ADD; or 123-LC-ADD; etc.
3) Exit: press and release STB once; check;
4) V9: you are complete; V8 and below; Dual Mode then Lock: press and hold ADD till light soldi, then 955.
Specific Boxes:
1) Xifnity: Some but not all Xfinity boxes require the OK signal. If this is your device, see above;
2) Dish: Some Dsh work better with the Enter signal, usually in Flipper's Mute button;
3) Spectrum: 50/50. I have seen boxes where the OK/Enter has no effect; others where it speeds channel changing for all except 4 digit channels, then MUte causes a menu;
4) Direct TV: usually works better with Mute.