A fantasy filled with magical creatures? A sci-fi futuristic covered with aliens? Anything is possible! Just start with your desired theme and build from there.;
, If it's a fantasy filled with goblins you cannot have an alien come out and attack the main character! You need to set laws, like who can and can't do this, where something will always be found, what is taboo and what not. Consistency is needed. You also need laws of Physics; not just government laws.
 What kind of creatures will be found in this world? Where will they usually live? What are their customs and traditions? What do they look like? Think carefully while making each individual race, this is where some of your laws come in.
 How do your races live together? Do they work as one, have a king rule over them? What language do they speak? What calendar do they use? Are they mostly harvesters? Are they religious or are they aggressive (wars and battles could be frequent)? What are their mating habits; what are their family structures? You can go with something traditional from our own world, or create something entirely unique!

, While variety is good you can't have half the world as a desert and the other as icy planes. In certain places that might be used a lot, (the capital city or the main character's home?) try to plan out where most things are and maybe even some of the residence.
 Think about what type of world this is and would these make sense. In a sci-fi world humans may be killed on sight or in a fantasy world only high-borne nobles may be allowed to practice magic. Make sure these have a point in the story.(If all the characters are going to be rich what is the point of that law?)

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What sort of life would a normal person (average or most commonly found) live in this world?
If I were someone from a different world what impression would I get when seeing it for the first time?
What sort of knowledge would normal people in this world have?

, Your world should be like a puzzle, when brought together correctly it should fit easily to create a perfect picture. From here, look back and decide what to maybe edit or figure out how this world in general will have its toll on the plot.
Figure out what kind of world you might make and see if it fits in with your plot.
Set rules and foundations for your world.
Invent or recreate races for your world.
Create a society.
Plan out what your world will look like.
There may be some elements you want in the story that are important.
Ask yourself questions about this world like:


If I were a traveler passing through and saw >insert race here< in their natural habitat what would I see?,
Put it all together.
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