Always choose the most populous neighbour. When left with no neighbours go back to the previous country and follow with the next most populous neighbour. Start with the world's most populous country.
Not counting the Gibraltar border, Bahrain bridge border, or dubious India/Sri Lanka border
Because that's how the quiz works! Estonia is answered after Latvia, after which you have to backtrack to the last answer that still has unnamed bordering countries.
That was a tough one for me. I got it eventually, but it took me a while to remember North Korea has a border with Russia. Great quiz overall though, nominated!