If you are already using assignment group weighting, you just value each assignment differently. Example: 30% of the total grade is quizzes. Yoiu can make a quiz worth 50 pts or 100pts. The 100pt one will be weighted double the 50pt one.
I'm not quite sure from your question whether you're talking about how to assign weights for the course or within an assignment group.
If you're talking about specific assignments within a single assignment group, then firstname.lastname@example.org gave the answer -- you weight them using the points of the assignments. Canvas computes the grade for each assignment group by taking the number of points scored divided by the total possible points for each assignment that is scored. If an assignment is excused or not submitted, then it is omitted from the scoring.
If you're talking about what weight to assign to the entire assignment group, then you go to the Assignments page, click on the options button (3 vertical dots) and then click on the Assignment Groups Weights button. Then you'll be able to set the percentage for each assignment group.