Prepare a rubric. When making a rubric separate each area of evaluation --- for instance, if grading a writing assignment have a section for style, grammar, and content. Choose criteria for evaluating each section, perhaps by using an example of an 'A' paper, a 'B' paper, and so forth.
Assign each section of the rubric a number score value --- for instance, out of 10 --- and total the score at the bottom of the sheet. If you have five sections, each worth 10 points, then the total rubric score will be out of 50.
Convert your scores. Take any existing percentage grade and separate the total grade into sections --- assigning each section to a rubric box. For instance, if 20 percent of the percentage grade is based on grammar, assign it to the grammar section of the rubric. You may need to adjust the percentage grades if they do not match up with the new rubric point values.
Total your scores. Take the percentage score and find out what it turns out to be in point value. For instance, if a student scored 50 percent on grammar, and grammar is worth 10 points, then that student will receive five points for grammar. Do this for each section, add up the final total, and that is the converted grade.