Backpacking SEA- where to begin?

Hey! I’ve got between August-January at least and I want to go everywhere!! But where to start?
Also how easy is it going through borders by trains/busses and how do you get to the Philippines?

I’m thinking of either starting in Vietnam, and go south, or maybe Indonesia and go up? I don’t know, or the Philippines haha, also it it reasonable to also go to papa New Guinea?