It's pretty complicated, a few moving parts involved but here goes. First you need to setup the attributes on the variable (I'll use RFR Part Number)
So what this does is it tells the system to copy the "RFR Part Number" from the custom properties of the part or assembly to the custom properties of the drawing and then onto the datacard. It's important that you get all 3 extensions on the custom property mapping or it won't work.
After that's setup you'll need to check out the file and open it in SolidWorks with the EPDM add-in enabled since it's the component that actually does all the work. Probably best to rebuild, save and check it back in and the mapping should start working. If the value on the part changes nothing happens to the drawing until the next time it's checked out and opened in SolidWorks with the add-in enabled, then it should update.