Determine your number range. One to five or one to 10 are common ranges. Don't make the range too wide or you will not be able to define each point on the scale.
Create categories for aspects you wish to measure. If you are grading a paper, example categories could be writing, grammar and research. Make sure you can define each of the categories so that they are measurable and observable.
Assign the number range to each of the categories and rate the student's performance on each category using the scale. This allows you to drill down on the exact strengths and weaknesses of the student's work, which gives the student better feedback about their performance.
Sum all the scores and divide by the total number of possible points. This will give you a percentage that can be then used to calculate a letter grade (if desired).