Getting started:
     First you must pick the room you want your ferret to train in.  A good place to start is the room your ferret spends the most time in.  You must have a specific spot you want your ferret to come to.  Of course the best place is by the light switch.   Pick out your ferrets favorite treat.  Choose something irresistible to them (and healthy since you're going to be using it a lot).  Make sure you have time to devote to this, and are consistent for your ferret once you begin.  I have not tried this particular training technique, but it stands to reason that it would be easier than my other techniques and easier for your ferret to master.  One drawl back, is that you can only call your ferret to this one spot.  This is only useful in evoking your ferret to come to you in one particular room (although some ferrets can be taught to do this in more than one room in the house with lots of training).

Procedure:
     Stand by the light switch, and flick the lights off and on a couple of times.  If you're ferret happens to look at you or walk towards you,  show them the treat in your hand and sign to them to come to you.  If the ferret does not come, or if the ferret doesn't notice you or the flicking lights, then go get the ferret.  Pick up the ferret, carry them over to the spot by the light switch, put them down in front of you on the floor, then you may reward with the treat.  Repeat.  Repeat often, and always, always have that treat ready.

Goal:
     The goal is that when you walk into that room where the ferret is, to get them to run right to you with the flick of the light switch.  Hearing ferrets can also learn this.  It's a great habit for them to stick to for emergencies when you have to round up ferrets, or find them in a room. On a more serious note... think of how invaluable this would be during a fire.    

Remember:
     To always stick with your training.  Keep the ferret fine tuned and practiced even after he's matered what you teach him.  I learned the hard way that they do get out of practice and even forget.  I moved once, and got so caught up in the convenience of having a ferret room and less free roam of an entire home that I got lax in my deaf training.  The result?  Is that my deaf ferrets won't come to my signs now.  I have to begin all over again, and in a new atmosphere.  I'm actually thinking of starting with this technique first, before my other ones since its room specific, and going from there. I learned that if a ferret  has a larger house to roam in, gets too busy with all the distractions to come to me when I want him to.  So that tells me I need to be more consistent, more frequent in practicing, and have great treats!