Every manager knows the feeling: Someone turns in a piece of work you assigned, and when you look at it, it’s not the quality you wanted. How do you tell a staff member that work isn’t good enough and needs to be better?
If this isn’t a regular occurrence and is just a one-time happening, you should simply be direct about how your vision for the piece differs from what you received. Treat it as a communication error more than a work quality issue – because if it’s not part of a pattern, that’s the likely explanation anyway. For instance, you might say:
If you can follow this up by pointing to examples of work that did meet that bar, that will usually help people understand what they should be shooting for.
You might also lay out different bars for different types of work: “For emails that are just being circulated internally, it’s fine to write informally and it’s not a big deal if you don’t proofread perfectly. But for anything going to the public, we need to be rigorous about writing and proofreading; that’s where mistakes matter and reflect badly on us.”
The important thing in all of these cases is to be direct about how the work is missing the mark and what the bar you’re going for looks like, keeping the focus on the work itself and not the person. (That last part is the difference between “this piece needs to do a better job of communicating with sophisticated audiences” and “your writing is making us sound amateur.”)