A big question you might have is: how do I evaluate my subject? That depends on what your subject is.
If you are evaluating a piece of writing, then you are going to need to read the work thoroughly. While you read the work, keep in mind the criteria you are using to evaluate. The evaluative aspects may be: grammar, sentence structure, spelling, content, usage of sources, style, or many other things. Another thing to consider when evaluating a piece of Evaluation writing is whether the writing appeals to its target audience. Is there an emotional appeal? Does the author engage the audience, or is the piece lacking something? If you can, make notes directly on your work itself so that you remember what you want to write about in your essay.
If you are evaluating anything else, use your head. You need to try, use, or test whatever thing you are evaluating. That means you should not evaluate a 2005 Chevrolet Corvette unless you have the $45,000 (or more) to buy one, or the money to rent one. You also need the know-how of driving a car of that power and a base of knowledge of other cars that you have tested to make a fair comparison.
On the note of comparisons, only compare things that are reasonably alike. People don’t care to know how a laugh-out-loud comedy like National Lampoons Christmas Vacation compares to Citizen Kane; that is for a different type of essay. Compare comedies to other comedies and dramas to other dramas. That is what people are looking for when reading comparisons in an evaluation essay. However, keep in mind that comparisons are always useful in illustrating an idea and providing context. They give shape and clarity to often complex ideas presented in evaluations.
Whatever you are evaluating, make sure to do so thoroughly. Take plenty of notes during the testing phase so that your thoughts stay fresh in your mind. You do not want to forget about a part of the subject that you did not test.