One thing that's important is to agree on exactly how much he will cut off. If you say you want an inch off, ask him to measure out an inch for you. And you both need to remember that your hair will shrink up when dry and curly, so an inch cut off the hair could make it 2 or 3 inches shorter, depending on how curly you are.

If parts of your hair curl more or less than other parts, point that out to him, so he can compensate and make them come out even. (For instance, if your underneath layer is less curly, it should be cut a bit shorter or it will hang out.)

If you can bring pictures of what you want, that's really helpful.