The first thing to do is identify what exactly it is you're evaluating; this will determine the style of your essay. If your assignment calls for an evaluation of something you'd experience in your daily life at college, such as the cafeteria food, your essay might be a tongue-in-cheek review about what you like and don't like and how things could get better. If you're being asked to review a literary work by a renowned author like Shakespeare, however, your tone will be dramatically different. Whatever your subject for evaluation is, remember to keep the context in mind at all times to help the flow of your writing.
Once you've decided (or your professor has decided for you in most cases) what you will be evaluating in your essay, it's time to determine just how you are going to evaluate it. One way to do so is using the method of compare and contrast. This tool is handy in almost every subject area, as few things are one of a kind and existing alone without competitors or counterparts. A few examples of ways to use this method:
Another way to evaluate your subject is with help from other people. If you're evaluating something that's close to you, like how well the student services office plans activities for the students, do a quick survey of other students to gauge their thoughts. They could share with you something you don't know, like the fact that they had a horrible time getting guidance in setting up a fundraiser. If you're reviewing a new movie or an album, check out what the critics are saying. This extra knowledge will serve you well when it's time to write your essay.
If you're writing an evaluation essay about the work of Stephen King, and you're deathly afraid of clowns thanks to his movie "It," try to put that aside as you write. Any deep personal feelings or preconceived opinions toward your subject can taint your writing. Although your essay will naturally have a degree of subjectivity, writing that Stephen King is a horrible writer and a cruel human being because he scared you twenty years ago is not going to earn you a good grade, since the man is a critically acclaimed writer and a known humanitarian.
While you're at it, don't go the opposite way on the praise spectrum. Using words like "great" or "amazing" can also put your reader on guard. It's a natural reaction for a discerning reader to find fault with what they are viewing, and you want readers to develop their own synonyms for the subject(be they great, amazing, horrible or cruel) themselves simply based on your evaluation, not by you saying those words first.
Occasional, off-handed "beautiful" or "exciting" is all right; just don't expect your readers to be convinced unless you make them feel that beauty or excitement. Keep it professional, and happy evaluating!