Whatever you write writes dealing with a world, whether the story is about speaking on a stage, workouts in a gym or a realm you designed. Many authors base their writings in the known universe with known parameters, and some add magic or fantastical creatures. Some might prefer to create their own universe. The late Jim Rohn said there are some things you just accept. Like, why does spring follow winter, or the sun rises in the east? When you are God, you make the sunrise where you want. We are going to play God and create a world.
There are several ways of drawing your world and I have used the three most popular ones. The first and the oldest is pencil and paper, whether it is freehand or tracing. Sometimes I used an opaque projector to shine a section of land on a wall and my paper. When computers came out I used a drawing program to redraw the map, but it used pixels, which distorts when zoomed in for a better look. Recently I started using vector mapping, which allows zooming without distortions, but takes longer to learn. Choose the method that is best for you.
Using Earth as the setting is easier than creating a new world. Designing a believable world with all its complexities can be a daunting task, however, when completed it can take on a life of its own. There are many possible starting points. Some will start large with the world, a solar system or larger. Others may start at the other end of the spectrum with an individual character and work to the large. I prefer to start in the middle and work both ways. The first thing I designed in my world is the city of Shen Sherin. After that, I worked on the surrounding area, before going back into the city and working on specific shops and buildings. For the course of this exercise, we will start large with a terrestrial sphere and its systems, including any moon (s).
Size of the world
Depending on the size of the will affect how much detail will make it believable. Smaller spaces will need more detail than large ones. A stage or tavern will need more detail than a world or universe. However, the small one can exist in the larger given both more believable.
Early in the design process, make the decision on how the physical laws work. Assuming that they hold true, then the planet size and composition will play a major role in gravity. Imagine going to a planet that a gravity one tenth of Earth, you could jump a long way. If you went to a planet with five times the gravity of Earth, you would find it difficult to move. The natives will be accredited to the gravity, but travelers may not. There could be adjustments to the planetary spin, orbital spin, and axis tilt that would alter the weather patterns.
The distance from the sun and the type of sun should play a major role in weather, the color of the sky, and even the atmosphere. There is no reason to start from scratch unless you want to, when it is easier to use a similar orbit as the Earth and just make changes to fit your needs.
There are several world generators on the Internet, many for free. They allow adjustments to the size of the planet, poles location, mountain heights and ocean depths. They will plot the weather, rainfall, temperature, rivers, and mountains. Some will allow the user to cut a section out and zoom in for more detail work. The programs are great if you do not already have something too specific in mind.