Don't try to jump straight into a magnum opus. Start with smaller things that result from an idea you have that's ready to turn into its own piece, rather than aiming to write a longer piece and adding inspiration into it after.
Everyone has inspiration. It comes in passing moments, and most people don't write it down.
There's a good reason why fan fiction became widespread, and that is because it allows you to take a premade setting and characters, and put your own twist on them without having to do the rest from the bottom up.