It has been asked before, but if you give some more information on the problems you're having, I'm sure some people can give you more specific advice. For now, I'll just expand on what PerhapsMan said, along with some advice. The best way I've found to implement a new piece of tech into my game is to put myself in more situations where I'm able to do it. Also, practice implementing new tech in obvious situations, and as you gain experience, you'll eventually be able to use it on the fly.
How to implement DJC Nair. First off, DJC Nair is interesting because it is slower than its more intuitive counterpart, SH Nair. The reward of DJC Nair is that it will allow you to be more safe with it, as well as gets larger follow ups after landing it. Keep that in mind. Some easy situations to land this include whenever you've missed a techchase, but you think your opponent will shield, at higher % Uthrow -> DJC Nair on spacies(they'll fly out at too high of %). Also try it after parry. I tihnk it is a solid option to try after a parry.
So...
How to implement parrying: I'd begin implementing parry by practicing on projectiles(Samus missles and Falco lasers), then I'd move toward parrying out of techchase situations (if you think Falcon is going to be a little slow on his stomp?), then eventually move on to parrying approaches(you can react to people jumping toward you). For these situations, you'll probably only want to parry -> jump, or parry into DJC Nair, until you're familiar with it.
How to implement DJC Uair: The best place to start is off of Fair on most characters. (Low % of floaties, mid % of average weight, high % on fastfallers) You can also work on implementing this after Dthrow on fastfallers, and it should start to become easier after you're familiar with those situations. To practice this (hand motion) I'd juggle computer spacies on FD. When you're ready to implement, start using more Fair in your friendlies, and trying to get these strings going.
As you get better, you'll start to be able to implement these different DJCs into your gameplay and can start to do more complex things like jump away -> DJC Nair back in, or fast DJC Uair juggles. Before you know it, you'll automatically using these with ease.
Note about DJC Uair: I'd avoid using this after jab reset or parry, until you're familiar with it. It is easy to miss the DJC Uair on opponents low to the ground due to it being narrow and above you.
