So I think I'm gonna pick up Yoshi this week, finally. The reason I didn't before is because I sucked (a lot) and could never get the timing for that DJC. My Yoshi is so bad I feel embarassed to use red.
What I am doing to practice the DJC timing is I essentially make the Nair have fixed knockback: I lock the dummy's % and get my Nair stale.
That way, I can always get the same knockback and hitstun. The same could be done with Uair chains, etc.
The first practice I am doing is vs Samus at 21%. This gives me 31 frames of hitstun for each Nair, and Samus won't tumble and fall if I miss. This is useful because you could set a comfortable threshold so a beginner could get started and gradually become faster/real.
Ultimately the goal is to be able to do that consistently vs Samus locked at 8%, which will give me 25 frames of hitstun, the same hitstun you get when you connect a Nair at 0% (vs Samus, Falcon, maybe others.) This method and this number (25F) is also relevant to shield breaking practice.
I still haven't a legitimate way to practice shield breaks by myself. Against CPs is annoying and not reliable. On versus mode it's useful if you're a beginner: assign your A input to the opponent's shield input -- so when you attack, they'll shield. This is even more unreliable because there'll be quite a few frames where the shield will be up needlessly, but useful to get the hang of it.
The good part (?)
A fresh Nair deals 14% and 26 frames of shieldstun (28 for those cheating Japanese players). And if I'm not mistaken, shieldlag and hitlag for Nair are the same (9F). Which means if you can keep the combo meter going, you're pretty much guaranteed to shieldbreak (no timing adjustment needed like when shine-canceling).
The downsides being: you're practicing without visualizing a shield, and when doing shieldbreaks you need move less than in a Nair chain.
Yes, I'm using an external program (cheat engine) to enable that. You just need another emulator (mupen rr) and the Madao table. It's very easy to use, you don't even need to know what 'byte' means.
edit: If you're interested, I can guide you through it.
Getting closer to Yoshi's feminine side