A self-evaluation is really just a process of gathering evidence to support your claim for a particular grade. Evidence can include written work (essays), demonstration of completing the work (like reading or quizzes), class discussion/participation, and anything else you think deserves being evaluated.
The best place to start when writing a self-evaluation for any class is the course syllabus. It should contain a lot of information that is likely to be helpful for evaluating your work in the class, whether that is from the teacher’s perspective or from a student's. Look for the requirements. Find out what you were specifically asked to do (for example, read the book and write essays for each chapter, which you did). Make certain you did them all.
Now look at the “extra” things which your teacher has probably included somewhere on the syllabus and which deserve consideration for a final grade. This might include things like turning in your work in a timely manner and participating in class discussion. If these are part of your teacher’s grading considerations, they should be part of your self-evaluation, as well.
Finally, look at the grading scale. This should not only give you the exact numeric breakdown for grading (i.e., 95-100% is an A), but it might also give you some guidance in terms of what constitutes “excellent” or “superior” work for this teacher in this class.
Whether you find these things in the syllabus or not, they are all valid considerations for your self-evaluation. The only thing left, then, is to assess the level at which you performed your work in this class. If the teacher gave you any graded feedback on your essays, quizzes, or anything else associated with this unit (or whatever the self-evaluation covers), that's a good place to start. If you have no graded feedback, you'll have to evaluate yourself based on the class standards set in the syllabus.
If you literally have nothing specific with which to measure your written work, a general standard will work. Is your work superior, excellent, good, fair, or poor? I've attached a generic writing rubric, below, to help you do that with your writing if you need that kind of help.
So, the first step is to check the syllabus for any clues about how your teacher is evaluating you, and then apply those things to your own work.
Second, if you have to find a way to evaluate your written work, use the rubric below (or a similar tool) to assess your written work and do so as objectively (fairly) as you're able.
Third, consider any other relevant factors, such as class discussion, timely submission of work, quizzes/tests, or anything else which might be applicable.
Finally, once you have gathered all the “evidence,” make your case. Be honest with yourself and your teacher. If your work was not superior, for example, don't try to claim that it was; on the other hand, don't sell yourself short and claim your work was only good if it was, in fact, excellent. Your instructor already has a sense of the quality of work you've been doing; this assignment is generally designed to make you think about how you're doing on your work.
When you write the evaluation, make your case by listing the evidence (written work, class participation, reading, turning work in on time, and any other considerations listed on the syllabus) and finish with a statement about anything you might have discovered or learned through the work you did, as well as through writing this evaluation.