Inspiration can come from obvious sources, like stories and characters we love that resonate with us and want to make us create an equally impactful world of our own. But I find this to be more of a motivator than a true source of inspiration. Sometimes it's helpful to revisit the stories you love when you're feeling uninspired to remind you of why you fell in love with your craft. Though in my opinion, the best place to find an idea is in the real world- but filtered through a different lens.
Dreaming in the shower of what you coulda/should/woulda done in a situation? Make it into a story. Looking at the news and thinking "Oh, god...it's the end of the world..."? Well, what would happen if it was? What if [insert historical figure here] was in the modern day? One thought can lead to another thought and then another until you have a brain tickling concept that usually turns out to be a very different beast than the train of thought that inspired it.
Brainstorming is one of the first steps to writing. You write out some ideas and do your best to flesh them out in a tangible way. Daydreaming is the more fluid precursor to brainstorming. The idea isn't on paper, it's just floating about on our brainwaves, cruising along on a current of thought and seeing what happens. We're told for one reason or another not to let our thoughts wander and to force ourselves into linear focus. To follow a 1-2-3 process. To write for X amount of minutes a day. To meet this deadline. To fulfill this word count. Daydreaming is free of all of this, and can often feel counterintuitive and unproductive for that very reason. It's okay to just let your thoughts drift off for a bit. I feel as if a lot of creative people don't give themselves the space to truly ebb and flow.
Admittedly, you can't daydream forever. At some point, you have to allow the seed you've planted to take root and then put in the work to nurture it into a full composition. And the way you choose to nurture that is up to you. Some work best giving their creation constant attention. Others work best by working in bursts and stepping away until they're ready to revisit with fresh eyes. Both are equally valid. A piece taking longer than expected to complete doesn't make you or the work a failure. I personally believe that there are times when trying to complete a piece is so difficult because there's more you have to learn, intellectually or emotionally, to bring it to fruition.