We generally use different drivers around here so I can’t help you with the correct voltage.
Make sure they are facing the right way on your board, quick image search will help.
The drivers and ramps do not need to be connected when the firmware is flashed, be they can be.
Make sure your power is only coming into the green plug on the ramps board.
To adjust the driver pots, the ramps has to be powered up, but it does not matter if the usb is connected or not.