To jump cancel up smash, input a jump, then an upsmash immediately – at most 2 frames – later.
This allows you to perform up smashes out of any state from which you could jump: out-of-shield, out-of-shine, and most ludicrous of all, out-of-dash. Learning this singular technique is one of the largest power spikes there is.
Whenever the discussion of Fox's best move comes up, up-smash is almost never excluded from the top three, and there is a very strong argument for calling it his best move outright; the legendary KJH agrees.
If you try to jump cancel up-smash with a standard grip, you'll quickly realise that your thumb cannot be in two places at once. You need to either enlist the help of another finger (“claw”), jump with the analogue stick (“double stick”), or skip the C-stick altogether and use a chargeable smash attack (“up + A”).
At the time of writing, there is still an active debate in the community about the fourth method, so I'll forbear to recommend or disavow it, but you should at least know of it. Some players use controller modifications to jump with the Z button (“Z jump”) – and usually also enhance the Z button itself with a tactile click – to get the same benefits as claw without the hassle.
You can do running up-smashes with a Run cancel, but this is a newbie trap. It is slower and does not work out-of-dash.