I've seen parody videos, I've heard other writer's accounts, but I can only speak on my own behalf. So, please, do not take this as gospel. Here's how it usually works for me:
During a nap, or at night, something will sometimes come to me in a dream. In these instances, I usually dwell on the idea that popped in. One idea I had was so good I shot straight up in bed - which sucked for my equilibrium, as I was still asleep - and grabbed the closest piece of paper and pen. I started to write the book, but decided to focus on the Hunters series for now.
Okay, so how did I come up with the Hunters series? Have you ever watched a news cast about someone who killed a friend or family member? It's always the same thing - "They were such a good person. They loved their family and would do anything for anyone. I just don't know what happened." Well, the husband and I were watching just such a story and I sarcastically replied, "Yeah, great guy. I'm sure he's possessed." That's when the bells chimed in my head. What if?
Think about it - What if someone you loved dearly was suddenly possessed by a demon? What if the only way to save their soul was by terminating their life? Could you do it? Could you thrust a sword into their body to release the demon's hold?
A majority of ideas that come to me are from dreams or what-if moments. My sister, friends, family members, etc have called or emailed me with great ideas. Problem with those is I can't "see" the characters in my head since they didn't represent themselves to me.
There's another issue: You can have a great idea, but there has to be characters to play out the scenes, as well as some great conflict and plot loops! Just because a story sounds good doesn't mean the characters want to act it out for you.
Do you write? Where do your ideas come from? Maybe you pull from the news, or maybe your own life.