Oh nice, you bought the controller AND the reservior. Excellent. First what I would do is take the reservoir off the top of the controller. Sit it down on something the same height. This way you can actually access the controller.

Next lets say you normally fill 10 gallons in your system. Then fill the ten gallons all through the controller (slowly) and add the nutes there. Then fill the rez with plain water.

You can check your controller as you like and when the rez is empty, its time to change out the nutes.