You can’t, really. There’s a sort of pseudo-Unfly Mode that flyers start out in, right at the beginning of a match. If you super jump right away, fly, and unfly, I believe it will allow you to attack or block on your way down. It gets lost easily, though. I believe doing it once loses it, doing any attack before you unfly loses it… Um, in other words, it’s basically useless unless you wanna bait an AHVB out of Cable right away to get him meter-less (a stupid Cable).
Otherwise, if you want Unfly Mode, you’re going to have to wait until you take a hit in the air… But it’s not like that’s hard. If you’re flying with Sent, and get hit by an anti-air (other than Psylocke), bam, you’re in Unfly Mode. If someone super jumps up and tags you once, bam, Unfly Mode. So many things put Sent into Unfly, that it’s not really an issue of getting into it, unless you’re against Cable.
Although I have experienced a flyer passing on his Unfly Mode to another flyer as he came in (i.e. Magneto gets un Unfly, and when Sentinel comes in, he’s already in Unfly Mode, without taking any hit). I do not understand this, though, and no one has been able to explain it to me. I tested it in training mode, but couldn’t get it to work at all. All I know is that it has happened to me many times, and I’m quite sure of it.