Luca, its generally an easy job i.e. no special tools required. The difficulty comes in the physical placement of the injectors, as Guru said. And of course make sure the dimensions of the new injectors are similar to the old ones so the geometry of the fuel rail isn't altered.
Grab a haynes manual, search online, or ask on an audi forum for more. I'd give it a 2 out of 5 for difficulty. 5/5 if you didn't leave the car running.