Create a scoring sheet with a scale of 1 to 5 along the top, and your grading criteria as a list down the left side of the page. Label 1 as "poor," 2 as "below average," 3 as "average," 4 as "very good" and 5 as "excellent."
Read the sample somewhat quickly, reviewing it holistically. Then read it again slowly and more carefully, making corrections if you wish, as you analyze the sample against your established criteria.
Assign a score for typos and misspellings, your first criteria. If you allowed students to use a dictionary while writing their sample, you may have "zero-tolerance" policy for even one misspelling. However, if you banned dictionaries during the exercise and you spot one or two such errors, you may be inclined to give the student a score of 3.
Evaluate the sample for correct grammar, including the correct use of nouns, pronouns, adjectives, verbs, adverbs, subject-verb agreement and pronoun-antecedent agreement, and score it accordingly.
Analyze the writer's use of grammatical conventions, including clear pronoun references, comma splices, fragments, run-on sentences and consistent and complete sentence structure. Grade the writer for punctuation, including his use of commas, semicolons, end punctuation, apostrophes, hyphens, dashes and parentheses. Assign a score.
Review the writer's stylistic choices, meaning his use of parallelism, coordination and subordination and varied sentence structure. Even if the writer relies too much on passive voice (or even active voice), do the sentences flow well and move easily from one thought to another with transitional words and phrases? Is the sample focused and well-organized? Give him an appropriate score.
Assess the writer's originality, creativity and voice. Was the sample interesting to read? Was he aware of his audience or merely writing to fulfill an assignment?
Add up the total score and assign a suitable range of acceptance, perhaps placed on a classroom curve, if you are inclined.