What is the most effective way to communicate with your instructor via evaluation-sheet?
In a word, constructively.
If you are overly negative, there's a good chance you will be viewed as a malcontent with an axe to grind. If you'd like to maximize the chance for real improvement, avoid blanket negative statements. For example, instead of:
This instructor was never available for consultation.
This class would be better if the instructor was more available to students.
(You send the same message, but with a more constructive – and therefore more credible – slant.)
Also, you should be realistic in what kinds of improvements you may expect to see. In my experience, some faculty take student evaluations very seriously, some don't. Some probably don't even read them; the forms are only completed because department policy demands it.
There's another important factor, too: the inmates shouldn't run the asylum. Some faculty regard student evaluations with disdain, and there may be legitimate reasons why. Sometimes professors who are easy graders get better evaluations from students, while more demanding professors get poor evaluations from students who are either lazy or else come to class with a sense of entitlement.
Truth is, there are a lot of dynamics that go into this. Just from reading your question, there's no way to tell if you're a good student who unfortunately had a bad professor, or if you're a bad student who got their feathers ruffled by a very good (yet challenging) professor.
A single evaluation won't have a huge effect either way, but, as I said at the beginning of this answer, I think your best chances for a positive impact comes by writing an evaluation with a more positive and constructive slant. In other words, try using the form to suggest improvements, rather than to harshly criticize. Such forms will be harder to dismiss outright.