Use the rubric to critique each person's (including your own) presentation to the class. Score each aspect of the presentation from 0 to 5 (with 5 being practically perfect). Add the scores to get a total out of 20.
It is of no benefit to give everyone the same score. Try to learn to be discriminating and to judge quality. A score of 3 on an aspect of the presentation is decent; a score of 4 is outstanding, and a score of 5 is practically perfect. For the total scores 15/20 would be considered a fine presentation. A 20/20 would be pretty rare as it would have to be practically perfect in every way.