I can't tell you THE correct way but I can tell you the way it works for me (with much more expensive cards, eg 1080ti)

One Power Supply for the CPU+MB (~300W)

Then more Power Supplys to power the Cards AND the Risers (same Power Supply for Card and corresponding Riser). So there is only data connection between the two PSU, no VCC so no difference current between two different 12V rails. That much caution may not be needed but I'm driving safe.