Hey Stefano - I wish I could give you simple answer, but this one isn't so easy. I think that the more falsetas you learn the easier it becomes to imagine new stuff, and that's because as you learn more you get more of a sense of the possibilities. Having said that, here are a few suggestions:
- Start with a structure/template that makes sense to you. Maybe take the structure of a falseta you know and change the melodies. I'd even start out with really simple soleá falsetas that have three beats of material and then develop that a bit just to get your feet wet.
-You can also reharmonize a falseta or idea that you like and see what happens. [One of my teachers in Spain is David Cerreduela, whose father, El Nani, was one of the greatest guitarists of his generation (Paco de Lucia said he could have been one of the very best but he became a preacher instead...). Anyway, David takes his dad's falsetas and updates them with newer hipper harmonies and it's really cool to see and hear.]
-As for jazz harmony, some of it works and some doesn't. In the end it's up to your ears to decide what works for you, of course. I find that a lot of the more common Brazilian voicings work great while the bebop vocabulary doesn't do it for me in flamenco, but that's my ears. Don't be afraid to try new things!
-Let us know how it goes. For me writing is the most satisfying part of it, so I hope you enjoy the process.