The most important factor to consider is the displacement of the first and second hit. The following image shows Up air done 5 frames after a double jump. The green area shows where the second hit is hits that the first hit does not.
|Up air 5 frames after Double Jump|
You can see how there is only a sliver of green. That is the only area you can hit with and get the single hit up air. Now, by doing the up air instantly out of the double jump, Fox will be moving up more quickly between the hits and create some displacement. You can see the difference instantly up airing after the double jump makes.
|Up Air 1 frame after Double Jump|
To take things one step further, drifting left or right will further displace the hitboxes. Fading back creates more displacement than fading forward, so for hitting an opponent thrown directly above you, jump slightly forward, then fade back.
|Instant Up Air, Fading Back|
|Instant Up Air, Fading Forward|
If the opponent DIs away, you will have to run towards them and double jump up air. The running start actually allows you to get even more displacement as you can see here.
|Run Double Jump Forward, Instant Up Air|
The running double jump instant up air is more difficult to execute because you need to hold the control stick the direction you want to jump during the jump. If you are just going straight up, you can use tap jump and then hit A to get the up air to come out instantly after the double jump, but if you are going to the side, tap jump is ruled out because you cannot hold to the side and up at the same time (obviously). So you really need to use a jump button and the C-stick. For this, I started using a pseudo-claw grip. During the start-up of the throw animation, I move my right hand into position with my index finger over X and thumb on the C-stick. Then, if they DI away, I am in position to land the up air. Using that technique is how I landed this single hit up air.
The other key to this technique is where you double jump. You want to double jump lower / earlier in your jump than you normally would, so that only the tip of the hitbox hits. Just invision the height of Fox's double jump below the opponent and double jump from that spot. I definitely found at first it was lower than I'd expect. Against Marth and Sheik in the low KO range (~90%), you need to double jump pretty low to the ground. It is simply a matter of practicing the spacing.
One last thing to note is you lose the extra 5% that the first hit does. That just means KO % is 5% higher with the single hit up air than normal, just keep that in mind.
That's all for now. Now stop letting them SDI out and hit your up airs. I hope to see more Foxes master this in the near future.