Click the Price Rules button from any interface (Product Explorer, Project Editor, etc.) and select "Selected" or "All". This will open the Price Rules form:
Click the [New] button:
Choose your Operation, Source/Destination Fields, and Value. In this example I am adding a percentage of 40% to my Unit Cost A (named Retail in this example, see Price Types) field to calculate the Unit Price A field.
You can uncheck the "Set price rules name automatically" option if you want to type in your own name for the rule.
If you wish to save the Price Rule for future use, click the [Save Rules] button.