I love that you're asking this, because we talk a lot here about how to respond to evaluations, and sometimes about how to get more useful responses, but not much about what students can do. I'm going to assume that your goal is to help the professor improve future versions of the course.
The most useful things you can do when giving feedback about a course are to be concrete and to avoid subjective judgements about pedagogy.
Concrete is probably obvious. General complaints (or compliments, for that matter) just don't give any information about how to proceed; things like "this professor is terrible" or "the lectures are unclear" are going to be ignored, even if the professor really wants to improve. It's a lot more useful to pick one specific thing and focus on that. Even if lots of improvements are needed, the professor is only likely to be able to make one improvement at a time anyway. "It would have been easier to follow lectures if the professor had given an outline at the beginning" or "There was so much material each class that by the middle of it, I was having trouble remembering what happened at the start" are a lot narrower than "the lectures are unclear".
By subjective judgements, I mean things like "there's too much homework" or "topic X is too difficult for this course". Most professors won't take complaints like those very seriously because we don't expect to agree with our students about what's too much or too difficult. Instead, you want to present more objective evidence a professor might agree with: "I was spending 20 hours or more a week on homework, and most of my friends were, too", or "when we hit topic X, I was studying twice as hard as usual, and still struggled with it".
Even if you want to mostly give positive comments, it's still a lot more helpful to point out specific things that helped you, because it makes it more likely the professor will continue doing them (or maybe even do them more or expand on them).