I use a very SHARP! knife. Not "sorta Sharp", or "was once sharp", and certainly not "is currently sHArP". SHARP!
I hold the head with my other hand as I do it, find the spot between two vertebre. and decapitate with a single draw of the blade. Finish with knife in one hand, bird hanging, head in other.
I do NOT recommend for others, its difficult to get right, and even more difficult for anyone even a little bit squeamish about the whole thing, Hesitation with this method is no courtesy to you, and it is CERTAINLY no courtesy to the chicken.
Assuming you have no stump and axe/hatchet skills, if you haven't the strength or the confidence for one of the common cervical dislocation methods, then I recommend a very sharp pair of bypass loppers - its not as clean a cut, but there is much less likelihood of failure, little need for strength, and it provides you a tiny bit of distance.