I don't understand logia guard

[Guomindang]

So by holding the button you become untouchable by most attacks, but how do I know which ones it won't work against? Some other logia seem to be able to wound me, but does all of them? Does e.g. Chopper's "Guard Break" damage me when I'm in my logia form even though he doesn't have haki?

And most importantly: How do you use Haki? Is it the same button as Logia Guard? What if I'm a character that has both like Kizaru? Does he automatically use Haki and Logia Guard at the same time then?

[Reifu]

All guard break (on a xbox controller X+A and Y+B or on a ps4 controller Square+X and Triangle+Circle) moves break logia guard. Haki is used with the same button as logia guard. And to your last question: Yes, if happen to get attacked at the moment you're attacking while using a Haki attack yourself (as long as you continue to hold the logia guard / haki button) and while your haki meter lasts.