The following instructions come from the Canvas Online Users Guide. This is an excellent resource to get answers to many "how to" questions.
For grading with weights, you must set up your assignment groups (categories) and then you must assign weights to these categories.
In Course Navigation, click the Assignments link.
In Course Navigation, click the Assignments link.
Click the Add Group button.
Type the Assignment Group Name in the Group Name field.
Note: You must create assignment groups before you can assign percentages to each group.
Click the Options icon.
Click the Save button.
Click the Weight final grade based on assignment groups checkbox.
Enter the percentage weights for each of the different Assignment Groups you created [1]. The percentage weights you specify here will determine how Canvas calculates the final grade for your course. These percentages can be changed at any time and students will be notified when any changes are made. The weights will show up in the Gradebook for both students and instructors.
Click the Save button [2].
Notes:
Assignment group weights can include decimals.
The total percentage of all assignment groups can be set above or below 100%.