Most haircuts that miss the mark miss it in the first two minutes, not at the chair. Here is what is actually useful to say.
Say how long you want it to last
"I want this to still look right in four weeks" tells your barber more than any number. It changes how short they go and how much contrast they build in.
Talk in inches or fingers, not numbers
Clipper guard numbers only describe the sides. For the top, show the length with your fingers or say how much you want taken off. "Take about an inch off the top" is clear. "A little off" is not.
Mention your routine
If you air dry and never use product, your barber will cut a shape that works that way. If you blow dry and use paste every morning, they can leave more length to work with.
Bring a photo, with a caveat
Photos are genuinely useful for shape and contrast. Just be open to your barber telling you how it will differ on your hair type and hairline. That conversation is the whole point of the consultation.