Product review (5 stars): Easy install and it works great
If you are not familiar with working on furnaces and working around / with electricity, then hire a licensed contractor.  Safety first.  Shut off the power to the furnace before starting this installation.

I installed an Aprilaire 600M a few weeks back.  My issue was that the unit would call for water at the panel when the furnace was off.  This relay is the answer to the problem.  Easy install as you locate the common wire going from the circuit board to the fan and run it between the metal clamp and the relay.  The relay is wired between the valve on the humidifier and the low voltage transformer that comes with the humidifier.  The relay is lightweight so there is no need to secure it to any surface.  Problem solved and it only took around 20 minutes of my time.