ok, i went through what you are doing. i watched falcownage, and saw him quad nair somebody and think "HECK YEAH THATS AWESOME I MUST LEARN THIS!"
it is not as easy as it can seem. for higher end people, or even mid level players like i have become, it becomes second nature. for a beginner, however, it takes some practice.
my suggestion is go to training mode. use falcon against ganondorf. put ganondorf to 10%.
now the trick to learning this is to start at 2/3rds speed. good people may frown at this, but seriously it will help you get the feeling down.
with 2/3rd speed against ganon at 10%, go close to one of the edges (with him infront of you, and PLENTY of space to combo), forward B him
as he is on his way down, short hop nair. you MUST fast fall, and you MUST L-cancel. once you land, do what audreyh suggests and go forward a little bit, short hop another nair and hold back right away. you will pull back, and stay in the right spot. if you fast fall correctly, you will be slightly below ganon when the second kick connects. L-cancel again, and repeat.
keep doing this in 2/3rds mode until it is easy.
now comes the hard part, regaining your ability in normal speed. i guarentee you at first, it will feel like 1x speed is insanely and unnaturally fast, but you will get used to it, again. start exactly as i explain, and keep in mind if you were like me, you will probably fail for a few minutes. then you'll start to get the second one to connect. then the third. after spending a little time on this, you will be able to tripple nair to knee, and quad nair to knee (once or twice i quad nair'd to a knee, but the last hit was off the stage).
another thing to keep in mind is that doing is is just practice. high level people WILL probably DI correctly and escape this. HOWEVER, you have taught yourself to horizontally juggle an opponent. put the second nair in to the person and let them feel the knee, instead.
once you can do this to ganon, try other characters. mario/doc are good, yoshi works well, link is very easy to nair chain, bowser and DK both large and easy targets. marth is floatier and a bit trickier, but certainly doable. i've double nair'd to knee a shiek ingame, but everything was DI'd up, so im not sure that will work in practice.
anyway, good luck, and remember that being able to do things like this isnt going to happen right away. and doing it to a computer is very quite different than doing it to a live opponent.
