Best way to give constructive criticism to designers?

What is the best way to give constructive criticism to people in creative roles? That could include designers, copywriters, etc.

I know most designers are numb from getting color / font change suggestions or when the employer thinks they're a designer as well.

So, what is a good way to provide actionable feedback that creative people can use to improve their work?

Short answer: Have an ongoing conversation which respects their role as an expert.

Longer answer: The answer to this rather depends on the role of the creative and the one giving criticism.

Let's start with the assumption that the reason you employed a "creative" is two-fold:
1) you are not skilled/qualified to do the work, and
2) they are skilled/qualified to do the work.

and that 3) they have delivered something that meets the scope and matches the input you gave.

An average creative will have agreed a scope with you, and asked for your input. Now you're in no real position to criticise. Of course, you can tell them what you do and don't like, and explain why - but don't expect them to perpetually re-work at no extra cost according to your whims.

A good creative will have worked/talked with your during their process and allow you to give feedback on the way. That was your opportunity. Don't expect any significant re-work after that point unless you're prepared to pay extra.

A great creative will listen and then deliver you something and you have almost no say in the end result. You hired them, because you respect their expertise.