Practice hitting Pichu with rising Nair oos. If you can't, then practice without shield first. I'm only suggesting using Pichu because iirc he's shorter so the timing is stricter, but I may be wrong.
If you have 20XX, you can use debug commands to turn on infinite shields, and tell a P2 Fox CPU to pressure your shield with SHFFL Shine. Use this to get an idea of how long shieldstun lasts for after a shine (hint: the white grainy lines on your shield that look like a TV with no signal indicate shieldstun). Nair oos too early, and the shieldstun will eat your inputs. Nair oos too late, and you'll be vulnerable to the followup pressure. This doesn't account for mixups though (e.g. spacie delays the shine, hits different spots on shield, uses different aerials on shield, etc.), so that's something that has to be learned vs. a player. Shieldstun isn't something that can be reacted to - human reaction time is 12-20ish frames on average
(test here), and I doubt shine shieldstun lasts more than 10ish. You just have to get the timing down and react to either the start of the shine (visual/auditory indicator), or some part of the preceding aerial/wait (visual).
EDIT: Another thing to consider is using Z to Nair. Z is a lot easier to get rising aerials with, since you split the inputs across two different fingers, instead of just using your thumb. Only detriment to using this method is the consequences for Nairing too early (during your 3 frame jumpsquat animation). If you input A too early you'll do an empty jump...which is bad, but if you can get away with it if you're lucky. If you input Z too early you'll do a JC grab, which is the last thing you want to happen during pressure. TL;DR - Z is easier for some people, A is less risky if you flub it, both are viable options.
