Pour some of the ferric chloride into the container -- a sufficient amount to cover the plate you're planning to etch. Add a small amount of the citric acid.
Add small amounts of water to dilute the solution to a density of approximately 42 Baume. Float the hydrometer in the solution, and read the number at the surface of the solution. This will give you a reading in Be and allow you to accurately calibrate your solution. The reading will be most accurate at a temperature of around 60 degrees Fahrenheit, as specific gravity is affected by temperature.
Use the masking tape to make "handles" for the copper being dipped into the solution. Etch a few practice sheets of scrap copper until the red-brown of the original solution gives way to a more opaque, muddy brown. This is the ideal coloration to do your etching at, so switch to the etching copper to do your desired etchings at this point.
Continue using the etch bath until the solution turns a dark green-black. The solution will be extremely inefficient at etching at this point, so dispose of it carefully (copper chloride, which is generated in the etching process, is toxic).