Italian men are known for the signature way in which they wear their tie with Gianna Agnelli being the prime example. The double four-in-hand knot is great because it adds a nice heft to your knot without bordering on the absurdity that can be the Windsor (or sometimes even the half Windsor). To tie this knot all you need to do is wrap the wide part of the tie around twice as oppose to once with a typical four-in-hand knot. It also helps to have a nice little nonchalance with the double four-hand, so try an imperfect dimple (pretty much standard with a four in hand anyhow) or an askewed tail. Both are great ways to add that just the right amount of Italian sprezzatura.
Basically my move goes something like this: I start off with my standard F/W kit, which is a button down collar oxford, tie (usually stripes or pin dot), flat front chinos and some wingtips. From here I simply knot up the tie in perfectly imperfect Neapolitan fashion. The end result is my normal "uniform" with a little extra heft and added style. I've even been wearing more spread and cutaway collars to take things up a notch, or two.
For more instruction and information check out some pics of Agnelli rocking his tie, an old GQ Rules video detailing the process of tying the double four-in-hand knot and, of course, the handbook on Anglo/Neapolitan style.