How do I add a rubric to a discussion topic?
Note: In order to attach a rubric to a discussion topic, you must first create a gradebook item and attach the rubric to that item. Then, you may select the gradebook item with rubric attached to associate with your discussion topic as illustrated in the steps below.
Important: The max point value for an activity that you want to grade with a rubric should match the point value for that rubric. A rubric’s point value is the sum of the max point value for each criterion in that rubric. If you grade an activity with a rubric whose point value does not match the max point value for that activity, then students’ point scores may be arbitrarily distorted.
Go to Discussions.

Select the Discussions tool from the tool menu of your site.
In the Grading section, choose the Gradebook item to associate with this topic.

Note: You must have an existing Gradebook item in order to associate the topic with that item, and the gradebook item must already have the rubric attached. If there are no existing items, you will need to go to the Gradebook to create the item first.
Preview the rubric.
Once you have selected a gradebook item with a rubric, you may select Preview Rubric to see what the selected rubric looks like.
Note: If you want to select a different rubric, you will need to go to the Gradebook and modify the gradebook item to use the new rubric. If you need to make changes to the rubric itself, you will need to go to the Rubrics tool and modify the rubric criteria and ratings.
Select Save when you have finished setting up the topic.

Your topic now has a rubric attached.
Topics with rubrics attached will display a rubric icon next to the title of the discussion topic.
For details on how to proceed with grading this discussion topic, refer to the article, How do I grade a discussion topic using a rubric?