It's an harmonic technique usually associated to movies soundtrack, so games and tv series used it too. You just have to harmonize the melody using the mediant relationships (commonly diatonic or chromatic).
Just count the 3 steps of the scale:
Em (mediant = G) -> Gm (dominant = D) -> Dm (mediant = F) -> Fm -> (subdominant B) -> Bm (mediant = D) -> Dm
As you can notice, there isn't exactly a strict rule. It's a soundtrack cliché, you use major chords to "add bright" and minor ones to "add darkness". Sometimes, you can jump to dominant and subdominant (this happens in this music), completely at your will. Hans Zimmer uses this a lot. I made these days a little piece to exemplify this to some colleagues: http://www.4shared.c...a_mediante.html
The harmonic sequence from my piece: Dm –> Fm –> Ab –> C –> Em –> Gm –> Bb –> D