Select settings then Traditional Grade Calculations
Select class you wish to set weighting for.
Choose actions by grading period
Choose category weighting from drop down and click the "+" sign for as many categories you will be using in your weighint formula.
Choose Attribute "Category" and type in the weight, in this example 80% and 20%.
Click SAVE when done.
Select other grading periods and repeat following step.
Select other classes and repeat all steps.