Can anyone help me create a math rubric?

In my math methods cl, I was asked to give a cl of 2nd graders an extended response question and to create a rubric to ess the students understanding. My teacher never explained to us how to create the rubric, and I have never had the opportunity to make one. She gave us an outline for a 4-point rubric with 3 headings (math knowledge, strategic knowledge, and explanation). I have no clue how to get started, can anyone help me?