I think writing fiction is much more difficult than writing nonfiction. You need to create characters, timelines, settings, and details within an overarching narrative. And then, you need to make sure it works, meaning there are no plot holes or discrepancies in the details.
My first piece of advice for fiction writers is to read for pleasure. Pick up a book and read every day. But, don't dissect every sentence or chapter. Instead, read the book for the story and enjoy it. When you finish a book, think about what you liked and disliked about the story. Doing this will help you become a better storyteller, which helps you writer better fiction. On a side note, I now write short book reviews for every book I read on Goodreads and post the reviews every month. The book review process takes very little time, but really has centered my focus on what I like in a story.
My second piece of advice is to outline your story before you start writing. When I say outline, I don't mean write out every single thing which will happen in your novel or short story. I do mean write a rough outline of the major plot points and conflicts. This allows room for creativity as you write without getting stuck in the tenth chapter because you wrote yourself into a corner. I started off as a pantser, but have moved on to outlining, which is a more efficient method of writing.
My third piece of advice is to use character timelines. If you are writing a novel, I am sure you have a complex storyline with many characters. The timelines between these characters must add up and make sense. So I use timelines for the major characters to help sort through ages, dates for major story events, and backstory. It has helped me tremendously.
When you are writing, put yourself in the story. Visualize the scene and the characters and write what you see. I can see my protagonist in my novel, Rachel, and her reactions to tough moments because I remember my reaction and others' reactions to tough moments. Put yourself in the story and write.
Finally, carry a small notebook with you (or use an app) to take notes about life everyday. If you meet an interesting person, write a small note about what made he or she interesting. Story ideas and new characters pop into my head often when I am not sitting at my computer writing. If I didn't write them down when they came to me, the ideas might get lost in the shuffle. Watch the world. Observe. Take notes.
Honestly, I wish someone told me these five pieces of advice (especially 2 &3) before I started writing my first novel. I probably would have finished the project six months earlier!