For this one, I'll start with the basics and then give the details for fanfics

- DO YOUR RESEARCH. The internet is an amazing tool and there are all kinds of tips and information everywhere. Quora, Reddit, google overall. Save it ALL. Do you remember the doc you had to start? Put all the important info there. Create a folder and save any image or song or article that you think it might help you on the road. This applies to every. Single. Tip.

- Genre. Is it a romance? A thriller? Scifi? Adventure? Self-discovery? Of course, you can mix it up, but choose a main one and don't let the others steal their spotlight. An example is Host, by Stephenie Meyer. It started as scifi and then there was beautiful discussion on humanity and its flaws, to our worth on this planet and then.... The romantic triangle (or quartet?) trampled it all. I'm sure you can also think of a book where this happened.

- Choose a time and a location. Present, past (which past? Victorian era? The 60's? Medieval?), future (in ten years? 50? 200 years?). The city you live? Your country? In another continent? Is it a small village or a metropolis? How is the architecture in that place? The technology? The economy? Social class? Politics? Climate? Seasons (I now live in Italy and people think it's very weird that in Brazil a mass of people go to the beaches for New Year, because there is summer. So yeah, the seasons change a lot the costumes)? Is it matriarchal or patriarchal? People are friendly or closed off? Do they move by metro or car? Is there more than one location? How far they are from each other and how long it takes to travel from one to another? Is there more than one language? What is a crime there? When is someone considered of age? People usually go to university away from home or close by? What names are considered common? And unique? And weird?

- Is it a new world? Darling, get ready to write a LOT. You can search on the internet the tips for world-building and I assure you there are a LOT. Is it capitalist? Socialist? Eco-friendly? How does the economy work, what are the coins? Is it one continent or numerous? One big empire? Tiny countries? Religions (yes, plural)? Gods are real and present or mostly believed? What's the country's history? Was is always like that? Is it a new country? It was part of an empire or another big country and then split up? Is there a king, a president, a parliament, a council, do people vote? How is the military? And the animals? The technology? Is there a moon? A sun? More than one? How many days a year? Months? Weeks? What are the names of the days? An AMAZING example of worldbuilding is The Name of the Wind by Patrick Rothfuss. I don't remember the details but their 'week' was like, 11 days. The first six had similar names (like MonDAY, TuesDAY) and the rest of it was completely different, because back in the day there was only the first six but then government, civilization changed and they added the rest of the days and had to give new names. Legends? Heroes? Historical figures? 

- Is there any magic? Well, get ready to keep writing a LOT. Check for advice on the web, but before trying to write, you have to create your magic system. Put limits on magic, put prices. Is it genetic? Random? Destiny? What kind of magic are we talking? Fireball or read the runes? Both? What is the difference? Is there a school for it? More than a school? Is it by apprenticeship? Is there hidden from the rest of the population? Is there a government control? Or an internal one? Are they considered saviors or disgraces? What do they need to do magic or potion or spell (wand, their body, fancy words, a familiar, spell book, intention, herbs, blood, lives)? Is there something external that sign them as magic (white hair, a birth mark, a missing or extra finger)? Do they have laws to obey (natural, magic or man-made)? Are there magic animals? Can they talk? Where does magic come from? Is in their blood? The Earth? Invisible particles around us? In Eragon by Christopher Paolini, the reason they can do magic is because along with gifiting their own energy (or energy they took from others) they use "the real name" of things. So if you know the real name of fire, yes, you can control, manipulate or create fire. In Rick Riordan's Kane Chronicles, if you know the real name of someone, you can control anything they do.

- Set a calendar. Do you plan to have everything happen within a year? A week? Thirty years? A month on the first part and then another month 250 years later? Sketch the main events, even if they are not directly related to your characters, as it might change the world around them. Your character is not a soldier, so they are not directly connected to a war, but it will change his life. Almost every country improved their airport security after 9/11, even if they weren't the one attacked. Dates like start and end of classes, business trips, a band presentation, holidays, games, celebrations, family traditions, exams, college applications, summer jobs, birthdays, anniversaries, etc. Check that calendar constantly to make sure you're not forgetting anything (like I had to create a new chapter because I had forgot about Valentine's).