To bantamize a breed, you can go about it two different ways:
(1), and if you want pure bantams with no other breed's blood introduce, essentially you would hatch as many large fowl as you can of the breed you want to bantamize (LF, large fowl) and select the smallest. People have done this for some breeds, like one making of Dominique bantams was created this way (sometimes several different people make bantams of the same breed several different ways over a period of time).
Note: This way takes very much time, but works, and keeps the breed pure. But again, it might take 5-6 generations or more before you get something significantly smaller
(2), is to pick a breed already in bantam form that you can get as close to the breed you want to bantamize as possible. For example, if I wanted to bantamize an Australorp, since they are so similar to Orpingtons, I would use an Orpington bantam. If I wanted to bantamize a Spitzhauben, I would use a Polish bantam.
You make the initial cross with your LF Faverolle on the bantam, then select the offspring the looks closest to your LF breed (if breeding a your Faverolle hen on the bantam, select a rooster, if using a Faverolle rooster on the bantam, select one or a few hens) and breed these back to the pure original standards (In your case, Faverolles). You then take those offspring and breed them back again, selecting the small ones that are starting to conform more and more with the Faverolle look you are going for. The more you do this the better, to rid them of as much of the foreign blood as possible. I would do it until you get 3-4 generations in a row where all the small birds you get look pretty much like faverolles, then you can breed them together.
Hope this helps