Dead heading is simply a matter of removing spent flower heads. This prompts the plant to produce more flowers, because removing the spent flowers deprives the plant of the potential seeds that evolution is driving it to produce.
The first step is to identify the spent flower. The problem is that once all the petals have fallen from a spent dahlia, the hard green part at the base of the flower (the calyx) closes over and what is left looks very much like a bud.
So unless you know how to tell the difference between the buds and the remains of the spent flowers, you can end up removing the very buds that you are trying to induce into flower. Fortunately, it is pretty easy to tell which is which when you know how. The buds are rounded and the dead heads are cone shaped.
Buds are rounded, a kind of flattened sphere of compressed petals, with the calyx (the hard green outer ring of sepals) fanned out below the bud.
Deadheads overall shape is pointed rather than flattened, cone instead of sphere. In addition the sepals of the calyx have folded up to enclose and protect the reproductive parts of the plant.
When you remove the dahlias dead head, don’t just cut it off below the flower. If you do, you’ll leave a flowerless stem that will just look ugly. Instead, cut back, with sharp secateurs, to just above the point where the flower stem joins a main stem.