Make your rubric. A rubric is a grading tool that breaks an assignment down into categories. Each category will receive a numeric rating. So, as an example, the categories of your rubric might be "introduction," "conclusion," "grammar," "argumentation" and "transitions."
Write a numeric value in each of the rubric's categories as you read each student's work. Typically, the possible numeric values would be a 5 for outstanding work, 4 for quality work, 3 for average work, 2 for below average work, 1 for incompetent work and a 0 for incomplete work.
Add all the given numeric values. So, using the previously mentioned rubric categories, say your student forgot to write the introduction and conclusion and had average grammar, argumentation and transitions. Then you would add 0, 0, 3, 3 and 3 for a sum of 9.
Add the maximum possible numeric values for each category. From the previous example, that would be 5+5+5+5+5, which equals 25 possible points.
Divide the sum of the student's given numeric values by the maximum points available. From the example, this would be 9 divided by 25, which equals .36. Any paper that is less than perfect should, at this point, be turned into a decimal that is less than 1.
Convert the decimal to a percentage. To do this all you have to do is move the decimal point two places to the right. So if you move the decimal point two places to the right on .36, it becomes 36 percent. Looks like you're about to break a student's heart.