The only two times you can really use the f-air without lag, if you're going to be landing after using it, are:
a) a full-height double jump. Full-hop, do your double jump at the very peak of the full-hop, and start the f-air immediately as you double jump. And I do mean "immediately" -- if you mess it up even the slightest bit, then Ganon's fist will get stuck in the ground, even though he finished moving his arm well before landing. It is... frustrating. Only works if you land at the same height as you jumped from (if it's below, you should get a few extra frames of leeway).
...and b) during a ledge-getup-jump. Hang from the ledge, press the "jump", and soon after start the f-air. If the ground you land on is at the same height as the ledge, you'll land without the horrendous f-air lag. (But if you ledge-jump a f-air onto one of the platforms on Battlefield, no dice: lag ahoy!) I don't know how much leeway you have before starting the f-air, nor do I know if Yoshi's Island and Pictochat screw you over here (as their ledges are slightly below the level of the stage).
c) is what Flying Dutchman said: use f-air off the stage for no landing lag on account of, y'know, not landing at all.
/RtEB
