How to describe a cut you will actually like
Most disappointing haircuts are not a skill problem. They are a translation problem — what you meant and what the stylist heard were not quite the same thing.
Why "a bit shorter" goes wrong
"A bit shorter" means something different to every person who says it, and a stylist has no way to know which version you mean until scissors have already made the decision for you. The fix is not more enthusiasm in how you describe it — it is more specifics: a length in relation to something fixed, like the collar, the chin, or the shoulder, rather than a feeling.
Bring a reference, but say what you actually want from it
A photo helps, but only if you say which part of it you are asking for. "This length" and "this much layering" are two different requests hiding inside the same picture. If your hair type or the shape of your face is noticeably different from the photo, say so out loud — a good stylist would rather adjust the plan before cutting than guess.
Be honest about maintenance, not just the look
A cut that looks right in the salon chair and a cut you will actually keep styled at home are not always the same cut. If you are not going to blow-dry it every morning, say that before it is cut, not after. A stylist working from an honest answer about your real routine will usually shape something that survives contact with a Tuesday morning far better than one shaped only around a photo.
Speak up while it is happening, not after
A cut is far easier to correct halfway through than after the last strand is on the floor. If something feels off as it is happening, say so then. Most stylists would rather pause and check than find out at the end that the last twenty minutes went the wrong direction.
Describing it before you book
If you would rather talk it through before you arrive, message with what you are picturing — length, how it should behave day to day, and anything you have not liked about a past cut — and you will get a straight answer on whether it is something to book.
Describe the cut you want