a google search for "phantom pilot training guide" yields the result as #1 straight from DJI's website
there's also a ton of discussion about naza mode on this forum but searching can be rough because just "naza mode" yields too many results.
Basically your phantom (I'm assuming a non-Vision model here but the principles are similar on Visions I believe) comes stock in Phantom Mode (Visions call it Vision Mode I think) where the flight computer is locked in GPS mode (auto-hover, hold position) and the 2 toggle switches on your remote don't really have any function (although you can program the right toggle to manually trigger RTH).
Naza mode is indeed a more advanced mode which opens up some other possibilities... they're more advanced so I don't know about this "training guide" but you should definitely have plenty of flights under your belt and be familiar and comfortable with piloting the Phantom before trying out Naza. When you enable Naza mode, several things are now possible:
1) the right toggle on the remote changes function. in GPS position the Phantom acts just like before when the computer was in Phantom Mode... GPS flight mode, auto hover when sticks are released. If you flip to ATTI mode, the Phantom will no longer try to hold it's GPS position when you release the sticks... that means from a hover it will just drift with the wind. more importantly, it means if you're moving laterally (let's just say forward) and release the lateral stick instead stopping on a dime it will act like a car when you release the gas... it will continue to "coast" forward. you have to apply your own counter-thrust (for example pulling back) to slow or stop the lateral movement.
Basically the Phantom is now drifting with the wind AND you have to apply all your own thrust to counteract any movements of the Phantom instead of the flight computer doing that for you. It's fun and not hard to master but hopefully you can imagine why it's not something an inexperienced pilot should be getting into. I personally do a lot of flipping between ATTI and GPS. When I want it to "hold still" for me I have it in GPS, and when I want more control and that other behavior I flip to ATTI. there's no harm in flipping back and forth, and the rear indicator lights show you which mode you're in.
2) it gets even more complicated... if you're in ATTI mode you can now enable the LEFT toggle on your remote to enable CL (Course-Lock) and HL (Home-Lock) modes. I personally don't use these but they do have value in certain situations I guess but they completely change the stick behavior... for example in home-lock, forward/back on lateral stick become "move away from" and "come back towards" the home point.. not simply forward/backward relative to Phantom so you should make sure you clearly understand them before enabling (via Assistant software) and using them. I humbly suggest you go to youtube and do your own research into course-lock and home-lock... videos explaining and demonstrating them are very easy to find.
Speaking of the rear indicator lights, they act MUCH differently in Naza mode. In my opinion
this video does a perfect job of explaining Naza mode blink codes and I feel like anybody going into Naza mode should watch it.
hope that helps!