How will Godzilla going to become Burning Godzilla?

Honestly, it's hard to say. There have been supposed "leaks", but I'm wary to look them up or believe them.

It could be something of a mix, whereby Godzilla gains the energy of Rodan and/or Mothra in order to absorb King Ghidorah's energy to power up even further and defeat him once and for all.

That'd be an interesting way to pay tribute to the previous ways Godzilla's powered up.

My guess is that Mothra will power up Goji. Goji will be grappling evenly with Ghidorah after getting nuked and upgraded, and Mothra's sacrifice will help him finish the fight for good.

Is it even confirmed at this point that burning Godzilla will appear? That trailer shot could just be Godzilla walking through flames. 

^No, it's not.

To the first or second G little "m" man? :P

The former.

Personally, I thought it was Rodan when I first watched the trailer. However, the ambulatory motions don't seem like they're possible for Rodan.

GMAN: Why not?

my personal wishful thinking is Godzilla just absorbs energy in general be it radiation and/or lightning/electricity and has enough to enter burning mode/critical mass and/or just make it something he can do naturally. some people think he is going to be mutated i'm hoping not i'm hoping he just naturally evolves/adapts similar but differently to say Goliath from evolve.