New to Creo - Parameter Roll-Up in Assemblies

Hello,

I am new to Creo and I am guessing this is a basic thing.  I have created a Parameter in my Part and Sheetmetal templates for Total Estimated Weight.  I have also created that same Parameter in my Assembly Template.  I have relations in the Part and Sheetmetal files that allow for adding weight manually if these are purchased parts, or welded, to had weld weight.  In my assembly I want to total all the "child" weights into the "parent" Total Estimate Weight Parameter, but I am not sure how to write a relation to do that.  Can someone help?

Thanks,

Mike