Should I create a World or use an Existing one?

Try both.
Ultimately, it is not possible to create a world from scratch, since you are always using what you know.
And what you know is the ‘Real’ world.
Try creating a world that resembles reality, & another one that would fit your ideal.
Eventually the 2 will merge into a 3rd one.
So, create 3 worlds.

Realism is not the End-Goal

Wherever you are Writing or Drawing/Sculpting/Modeling you want to find a balance between Realism & ‘Appealism’.
The Characters & Environments you will depicts will be Fictional.
Real-world Dialogues aren’t appealing.
Clean them.
The Characters will understand each other, to the point they will be able to finish each other’s phrase at time.
That’s not realistic.
That’s Aesthetic.
Almost always choose Beauty over Real.
That’s what the public needs to be engaged.

Real or Ideal?
Either way you will have to document yourself.
If you want to create a believable atmosphere you need to know about Thermodynamics, but also Climatology, basic Zoology & Botanic…

In the best case scenario, you’ll need at least 4 types of textbooks: Economy, Psychology, Ecology & Physics.
Lend 1 of each at your local library & learn the basics.
Once you’ve got this you can world-build comfortably.

If you don’t want to read textbooks & encyclopedia, at least in the beginning, you’ll need some basic elements.
What are they?
What is essential to this World?

If you are designing a settlement:
Who are the People?
- What are their main rules & preoccupations?
- What is their daily life like?
Who is in charge of the settlement?
- Who is the leader?
- Is she leading alone or as part of an assembly?
- Is she benevolent to her people only or also to foreigners?
- Is she gregarious or recluse?
How is the City/Town/Village?
- What’s her resources?
- What’s her origins?
- What’s her landmarks?
What are the Relationships between the different settlements?
- Faith-related
- Commerce & Trade-related
- Security & Military-Related
What are the main Organizations?
- Faith-related
- Commerce & Trade-related
- Security & Military-Related
How is the education system?
- Is it open?
- Does it favor specialists or generalists?
- Are there trainings included?
- What’s the family model?

If you are designing an environment:
What is Nature?
- How is it defined?
- What are the core elements of this world?
- What types of landmarks/landscapes are there?
How does Space-Time impacts nature?
- How is the day-night cycle?
- What is the effect of light on animals & plants?
How is the Atmosphere?
- Climate & Weather
- Temperature & States of matter
- Water cycle
How is the Wildlife?
- Prey-Predator relationships
- Relationship to Humans
- Pets & Feral

1 last thing, if you want to make a ‘Real’ World read Non-Fiction.

Now you’ve got knowledge of the ‘Real’ world you can think about improving things.
Look for research papers on improvement on a specific domain, like hunger or famine, & futuristic technologies that will help solve it.

If you are designing a settlement:
Who are the People?
- Write down a set of 10 rules
- They will be the foundation of your world
- What is your ideal daily life like?
Who is in charge of the settlement?
- How would you rule?
- Alone or as part of an assembly?
- Are you managing people or would you delegate this to someone else?
- Would you act as a chairperson?
- Is your world open?
- Or do you live in Autarky?
- Are you often outside & available?

How is the City/Town/Village?
- What’s her resources?
- What’s her origins?
- What’s her landmarks?
What are the Relationships between the different settlements?
- Faith-related
- Commerce & Trade-related
- Security & Military-Related

What are the main Organizations?
- Faith-related
- Commerce & Trade-related
- Security & Military-Related
How is the education system?
- Do you need a formal education system?
- Do you favor specialists or generalists?
- How do people learn?
- What’s the family model?

If you are designing an environment:
What is Nature?
- Do you have a round planet, a cubical, cylindrical or a flat one?
- What are the core elements of this world?
- What types of landmarks/landscapes are there?
How does Space-Time impacts nature?
- How long are your days/week/months/years?
- Do you use this system at all?
- Does your world use Euclidean geometry?
- What is the effect of light on animals & plants?
How is the Atmosphere?
- Does your world have unusual elements, other than the basic 4?
- Climate & Weather
- Temperature & States of matter
- Water cycle
How is the Wildlife?
- Is there a major threat coming from the animal world?
- Is there a major threat for the animal world?
- Prey-Predator relationships
- Relationship to Humans
- Pets & Feral

What matters is consistency.
Tap into the ‘Real’ world as much as you need to support your point.

What makes your People appealing?
- Why would someone spend time with them?
What kind of Relationships makes these people unique?
- How do they overlap with the World’s evolution?
Why would someone join 1 of your Organizations?
- What are the advantages?

How will you redefine Nature?
- How do you define Inertia?
- How do you define Life?
- How do you define Evolution?
Will you use Space-Time at all?
- Does your Planet rotate?
What type of unusual elements are there in your Atmosphere?
- Never-changing climate
- Fixed temperature
How is you exogenous Wildlife?
- Are there alien species?
- Are they humanoids?
- How are they considered/treated?