Set up your grading scale. Decide what type of scale you'll use to judge the piece of writing. For example, an essay could potentially be worth 100 points or 5 points.
Select the criteria by which you will judge the writing. Expository writing can be judged based on whether it clearly explains a given topic, how well the writer handles grammar, proficiency in diction and syntax and whether the piece is appropriately organized into an introduction, body and conclusion.
Assign a weight to each criterion according to importance. Organization and originality might be more important, and therefore worth more points on your grading scale, than grammar and spelling.
Assign point values to each criterion based on your grading scale and weighted importance. A sample rubric based on a 20-point scale might make ideas worth 10 points, grammar worth three points, style worth three points and organization worth four points.
Determine how many points will be deducted for errors. For example, an essay containing 10 grammatical errors might lose one of three possible grammar points.