Let me first say that while I occasionally sift through the Greninja forums I don't do so enough to know if this topic has already been covered but if it has:
Could someone please link me/give me tips on how to jab lock an opponent after a falling Nair -> Footstool?
I've been trying to incorporate more footstools into my playstyle as of late and seen that some players (PHaZeX3R0) usually locks with the spiking hitbox of DAir or am I wrong?
I imagine that you're talking about the Footstool Dair lock.
This has to be done at a pretty low percent and is dependent on the character. The combo itself is, Nair -> Footstool -> Dair.
In order to lock you have to Dair right after the footstool. But if you Dair too late it won't lock. However, if you lock them successfully, you can do it a second time (depending on their percent) by Shuriken Stalling, after you hit the Dair, and FF Nair -> Footstool -> Dair. I'm not sure if you can do it a third time but someone feel free to correct me if I'm wrong but this is just my understanding of it all.
Another cool Jab lock Greninja has is his Dair -> Footstool -> Bair. This is a lot easier than the first one and has to be done at a higher percent. (Not sure on percents. My guess would be 90% - 120% on most characters.). If you Dair your opponent with the spike hitbox on stage, they'll pop-up and that will allow you to get the fairly easy footstool into Bair.
Sorry if this is wonky or confusing. I kind of rushed through it but I felt that I could give some decent insight.