A Theory of Creativity
My theory on creativity: The brain does two major things above and beyond basic stimulus-response and regulating body function. First, it generates narratives to make sense of the sensory impressions that your sense-organs deliver. Secondly, it edits those into meta-narratives that tie groups of impressions together into heuristics. This is how we model cause/effect, figure out that calling someone a mean name will make them angry, and that people who look like X probably will act like Y. (So pushed too far, or left to go unchallenged leads to racism, sexism, xenophobia, etc.)
That meta-narrative meaning-making process is coordinated by the pre-frontal cortex (massive over-simplification) and is (mostly) shut down during sleep. During sleep our body is still receiving sensations and so the brain is still doing its narrative-building work, but without the benefit/detriment of the pre-frontal. This is why dreams feel like they make sense while you experience them, but on waking introspection you realize they don't follow many heuristic patterns your waking brain has constructed.
In my theory, creatively is a skill at regulating the pre-frontal cortex in a more nuanced way; to be able to slip further towards the dream-state mode of brain actively. Suppressing the heuristic-building/reinforcing analytic mode of our waking brain. This is theoretically what is happens during 'brain storming' where we say "no idea is a bad idea". And some people are better at embodying that than others. I think it goes to the skill of down-regulating the pre-frontal cortex enough to not let it stomp all over your dream-like juxtapositions.
If all of that sounds like nothing you've experienced than my best advice is to use outside prompts with random connections to facilitate brainstorming. For example, if you wanted to create a fantasy animal, make a list of real animals, then a list of plants, then a list of minerals, etc. cut them up, and mix them all together, then pull 3 things from each pile and see what the result is. (Iron Ivy Horse) (Quartz Oak Snail) Or if you want to make a character, make a list of attitudes, beliefs and professions. Or many other possibilities: phobias, passions, physical or psychological disabilities or strengths, etc. (One armed, bi-polar, jazz loving, atheist priest.) Of course, coming up with the lists of what you're going to throw together is a creative act in itself, but I feel like more analytically-minded people can accomplish it because you can observe the real world and break down what is actually there to generate the categories, then google them. eg. put "personality traits" in your search bar and write down the first 10 you see.
In summation: Creativity is the practice of down-regulating your pre-frontal cortex at-will to generate a lot of garbage juxtapositions, then up-regulating it slowly to curate the results.