If you want to argue for example, that "all cells have a nucleus", and you name a PhD biologist who makes that same claim, and all PhD biologists concur with the named expert biologist, then your appeal to authority is valid.
This serves as an example not only of a valid appeal to authority, but also to the types of claims that can be established by such an argument. They are basic, elemental, trivial almost, and noncontroversial. And when used appropriately, a valid appeal to authority never proves your main point, but a premise used in your larger argument. It is used as a shorthand for establishing one of your premises that you'd otherwise have to demonstrate through perhaps hundreds of microscope slides of various cells, each showing a nucleus present. Instead of that, you can make a valid appeal to authentic authority within that domain.
It is why it is said that the appeal to authority is the weakest form of argument, but the saying does not suggest that the appeal to authority is ipso facto fallacious, just that it is weak, which I agree with and I've not said otherwise. I'm pointing out the error in saying that all appeals to authority are fallacies---they are not.
As a counterexample, consider perhaps a PhD in a domain like engineering, and citing this PhD to establish something outside of the domain of engineering. In such a case you've committed the fallacy, because you cite someone who isn't an authentic expert in the domain or field or discipline in which your claim resides.
I admit that it's a little tricky, but not very tricky. You must name the authority to enable verification that the cited source is an authentic authority in the field in question, the person must actually be an authentic expert in the field (the PhD degree or other doctorate is one of many ways to do it, and is another handy shorthand for ensuring that your source is an authentic and authenticated expert), and your source must be claiming what all the experts in that field also claim, uniformly.
If the matter in question is controversial, and all the domain's experts do not agree on it, then a valid appeal to authority is just not possible. You're going to have to prove your point all the way down, rather than employ the shorthand of validly appealing to authority.
