Forum rating is a traditional grading method  that rates individual posts. To enable this, follow the steps below:

1. Navigate to your course

2. Turn Editing On

3. Navigate to the Forum that you would like to change

4. Select the Edit menu inline with the Forum activity, select Edit Settings:

5. When a Forum is created, the default grading method is Ratings. Use the drop down to select your preferred method of  aggregation. Sum of ratings is the most common and easiest for rating multiple posts in the same discussion forum. 

6. Enter the maximum grade you want for the forum if different than 100. Also you can select which category you want the forum in your grade book if you use categories. 

7. Lastly, scroll to the bottom and select a Save option: