Just write. You learn by doing. Crafting sentences and scenes and characters and worlds becomes easier the more you do it.
Understand the type of fiction writer you are. If you are not that fond of planning ahead, that's OK. If you need to plot every little thing in every chapter, that's OK. If you're in between, that's OK. If you are comfortable with how you write, it will be easier to do so.
Use timeline software. I write my stories, then go back and add everything to a timeline as part of my editing process. I've caught mistakes in dates doing this and can fix them before they become a problem.
If you're stuck writing, answer a prompt question. These questions can be serious or silly or informative. Prompts like, "Where's the coolest coffee house in your setting," or "What does the color blue mean to your cultures" or "What's the most terrifying carnival ride at the nearby theme park" may never make it into your story, but focusing on them can get you thinking about your tale, your characters, your world, in a different way--and perhaps break through that writing block.
Write down all names and places and a brief description of them. There's nothing so frustrating as trying to remember the name of a bit character that suddenly became more important, searching through the chapters trying to remember where you mentioned them, realizing where you thought you wrote them is not so, and re-reading half your book to rediscover them.