The flat-side step above is the whole game and worth expanding, because it is where people go wrong in a way that convinces them the method does not work.
When you work the scallops you push metal to the other side. That burr is thin and it will cut nicely for one loaf and then fold over and be worse than when you started. If your experience of "I tried sharpening a serrated knife once and ruined it" is that pattern, this is what happened.
Deburring is a light touch on a fine stone with the blade absolutely flat, or a few passes on a strop, or even drawing it through a piece of end-grain wood or cork. Almost no pressure. You are removing something the thickness of a hair.
One diagnostic before you start, since you say it tears rather than fails to cut: look at the points, not the scallops. On a bread knife the points are what break the crust, and they blunt long before the curves inside do. Hold it up to a light and look along the tips — rounded, shiny flats where there should be points is the usual finding on a knife that tears crust. That tells you the tips need the most attention, and it also tells you the knife is not worn out.
And never run a serrated knife through a pull-through sharpener. That is the one thing that genuinely destroys them.