This is a device I got from John Williams score to "Harry Potter", it's in all three movies. Check out the part that begins at 1:39 here.
It's similar to TD8#10 but where that device is more diatonic in nature, this one's signature is the fact that you get some chromatic notes in there. It works best over a pedal since that holds down the tonality so the chromatic notes won't sound out of place, but instead give that special feel.
Make sure you're satisfied with your melody and then simply experiment with the different triads below. Here you can try major,minor, diminished and augmented. This gives you 12 options per melody note (4 triads with 3 different notes) so it's a huge amount of combinations available. Take it little by little, bar by bar and don't move on until you find the right combination for each note. Knowing that there's a finite number is a big help here so you don't feel overwhelmed.