Authors find that actually planning the entire story, though it may take months, is beneficial to creating a smooth, connected story. Developing your characters is important, and conveying them correctly in your work is even more so.
For example: your main character, Mary, is very smart (according to your planning). Don’t write, “Mary was a very smart woman;” write, “Mary always stayed in the library for hours, picking through reference book after reference book, unable to decide what she was going to study today. " The latter lets the reader pick up on the fact that Mary is smart without you force feeding it to them.
For example: Harry Potter has a very fantastical setting, but the idea of school, politics, and the importance of friends and family are very real and very universal and make the fantastical elements feel more realistic.
Put people that you see into the same story, even if they aren’t interacting in real life.
Using what you’ve observed, write a story about them, using the mannerisms and descriptions that you’ve noticed about them.
Pick out the facts and things that most interest you and build stories based on them. For instance, you could take something as simple as a decommissioned lighthouse in Maine and turn it into an epic tale of good and evil.