Colleen offered a great answer to this question. You should be honest, constructive, and specific. And most evaluations are anonymous. I would only add that, when writing your evaluation, consider what things your professor could actually change and what will improve the class. Think about what you didn't like about the professor and why you didn't like it. Why did it make the class harder or less interesting?
For example, saying that your professor's clothing choices were ugly or that he was "mean" aren't particularly helpful comments. It's hard to change being "mean." You could, however, say that you found your professor unapproachable and dismissive when students asked questions. Those are things someone could work on that would improve the class.
Another thing to remember is that everyone is more receptive to criticism after you've said something nice about them. So, if you can think of any positive to start with (Were they knowledgeable? Was there an assignment you liked? etc.), it's not a bad idea to "warm them up" with this so they'll consider your comments more seriously.