Examine the student's work. Make notes or corrections to help explain to the student where she can improve. These notes also remind you of key strengths or weaknesses that you need to consider when you fill out the rubric.
Mark the student's performance on each area on the rubric. Refer to the notes that you made on the paper to guide you. Make sure to include additional notes on the rubric to clarify the problem, if necessary. For example, if you are giving a student a low score on paragraph structure but you wrote only some light notes on her paper, write some details on the rubric about why she scores low in this area.
Add up the student's score, if the rubric is a numerical one. Translate that score to a percentage by dividing the points the student earned by the number of possible points. If the scale is holistic rather than numerical, decide on a percentage and grade based on the student's performance.
Write the percentage and grade on the rubric and attach it to the student's paper. Pass the paper back to the student so that he may read your notes and use them to improve his next paper or assignment.