There's really no secret to this at all, its just practice. Go into practice mode. First just do them very slow- instant float, nair, drop. Do it without attacking the enemy. Then try and slowly speed up the rate u do that. You wanna get to the point where you are doing only nairs, and not accidentally doing any other FC attack. Than try to add in a fast fall once you get to the point where u stop accidentally dair'ing. The fast fall is the hard part, so be patient. Again, start slow, but still do the fast fall. Eventually u will get to the point where you can do fc nairs extremely fast because the fast fall speeds up the rate at which you can do them significantly.
Now its time to practice this on a computer- this is very important. I used to think it was important to just get to the point where I could FC nair as fast as possible...but that is less important than timing. Lemme explain-
When you hit a person with an attack, it delays the rate at which you fall by a little bit. When you FC nair someone, it takes a bit longer to get down to the ground than if you were to just wiff the attack and hit nothing. The same principle happens when you hit a shield. In fact, the amount of delay from hitting a shield and hitting the enemy is exactly the same. If you want to be good at FC nair as a form of pillaring (shield pressure) than you want to practice and get used to the timing that you can FC nair on someone's shield.
Here's how I do it- go to vs mode and change the damage ratio to ".5" Then change put on handicap and add bowser as a CPU. Put bowser on level 8 handicap(dont use 9, hes too strong), and put yourself at level 1. This allows you to practice FC nairs on bowser, and keep hitting him in quick succession without him flying away. This sort of simulates a shield. If you can get the timing down so that you are hitting bowser with extremely fast float cancel nairs in quick succession, you should be in good shape to nair pillar on someone's shield.
Once you get this down, practice adding in double and single jabs between your nairs, because this is extremely useful for the pillar techique. When bowser starts getting to around 700 percent, try practicing FC uairs in quick succession to get you better at your juggling.
TL;DR- now you're good at FC nairs
Edit: FYI- I use the bowser+handicap+damage ratio combo to practice and warm up my nairs before every major tournament. Its definitely a valuable form of tech skill practice.
since the heavy bowser training is so key to my advancement in tech skill, i even gave it a spot in my "trailer" combo video that i made a while back. This is how your nairs should eventually look:
http://www.youtube.com/watch?v=Z2FDvIf1JVI&feature=channel- It's right in the beginning
