How to begin.
The first task is to learn the left and right hand parts seperated, each at the same tempo, no pedal and with a metronome set to the downbeats, (beats one and three).
What is the correct practice tempo?
The correct practice tempo is the one that allows you to play the most accurately (and thus, artistically) without seizing up from going too slow or too fast. Yes, going too slow will cause you to freeze just as much as going way too fast! If you practice too slowly, you will develop a note-wise performance. You must go fast enough to develop a sense of musical narrative, as if you are telling a very interesting story to a good friend. You should be trying to tell your "musical story" as clearly as possible. The trick is to find the fastest tempo in which you are in perfect control, technically (of course!) but more importantly, aurally and musically! The correct tempo will probably be pretty slow, but never arbitrarily slow.
You need to develop a sense of gently rocking in the l.h., playing accurately without consciously pressing each key. Just concentrate on the bottoms and the tops of the arpeggios.
Do the same in the r.h., being careful not to go faster than you can maintain control while at the same time not going too slow or tensing up. Continuity is the goal here. Work on developing a seamless legato in both hands. Practicing without the pedal will help you find the minimum amount of energy needed.
It's a simple problem of alignment. (How to coordinate.)
When you can play the l.h. and r.h parts seperately, from start to finish without any breaks, you are ready to combine the two into one. Try to coordinate first on the common beats. To do this, play the first and fourth notes (the bottom and top) of the l.h. [---]tuplets while playing the complete r.h. You will develop a strong sense of structure that will later carry a strong performance. This way you don't have to master the tricky 3 against 4 rhythm before you begin making music. It is far easier to go from the general to the specific than from the specific to the general! If you properly align the common beats, the details take care of themselves.
To make an analogy...
It's like building a house, the foundation is first (that's the l.h.) the frame is second (coordinating the common beats) and the tiny detail of the roof tiles (the 3 against 4 rhythm) is last. Working first on the details of the "Fantasie-Impromptu" would be like building a house by decorating it before the foundation was in.
Coordinating the Whole
Your entire body should be flexible to allow fluid movement. Rock gently from left to right with the beat. This gentle rocking will generate the initial momentum needed to acheive alignment. All you have to do is transfer that momentum through your body and into the piano.
The keys to mastering this piece are coordination and energy management. If a part feels "impossible" stop, close your eyes, take a deep breath and tell yourself that it most certainly is possible. You just don't know how to do it yet. Patience comes in handy, here.