Thanks everyone. I paid attention to the soundttrack on other episodes and saw how the show's composer used different instruments and chord patterns, especially dissonance, to create a mood. Then I just pulled out some sounds from my synths and used them. I definitely needed strings and piano plays a big role in the show's opening and other scenes. Mostly I just went by feel, then listened, changed something, then listened, played it for my wife and daughter and got their feedback, then tweaked the cues and timing so it lined up better with each clip and with each other. Some things didn't work so they came out. Listened on headphones, then on speakers, adjusted EQ and compression, mastering levels and such. It's a process, but not terribly different from putting together any other song or recording.
A lot of trial and error, like most anything else you do for the first time.