As Henrik describes it is good.
To that I would add that you can hold z the whole time and tap A to throw which will ensure that you begin priming as soon as possible, and it will be consistent.
What I do for the juggles:
1. Hold z and look up at the same time, when you reach full lookup take stick pressure from 100% up to 80% or so (still at full look up), and throw. Holding the stick at 100% up does make the grenades go farther behind you, you can see that if you try it standing still. I find about 80% up on the stick is about right for agent. 100% will cause you to miss many first grenades.
2. I like to add a tiny bit of lookdown off straight up, maybe 1-2 degrees, and throw when #1 reaches peak.
3. Add more lookdown, a few more degrees, and throw when #1 lands. You want to throw 3 as late as you can.
Repeat 1 and 2 for 4 and 5. Don't throw a 6th because you won't hit it, it'll just add lag.
