First of all, you should be clear about how much time and effort you want to put into your campaign. Depending on how sophisticated and bigger your world is, the more time you have to put into it even before you start playing the story. My first story Aldrien I built up piece by piece during the game, so the first three capitals were already finished before, but the rest of the world has developed in the course of the story. Meanwhile, in my second story Eupharis, the whole continent including countries, politics, history and trade of the countries was already present. It is up to you how detailed and elaborate you want to make your world, here I give you the detailed version of world building to hand, which you can then shorten according to your own needs.
Concept of the countries
In the beginning it is not yet important to have the perfect map. Think about what countries you find interesting, write down the ideas for the entrance to them. These ideas can be based on cliches or directly contradict them. Consider which people predominate in which countries. Which countries are especially powerful? Which countries are in conflict and why? Are there allies or alliances? Which countries are located more on the equator, so are very warm and which are more towards the poles and are rather cold. Think about whether you want to have your countries separated by sea or have them all on one contiguous land. Here a sea offers many possibilities for ship actions, hidden islands and pirate life. You can also orient certain countries to certain classes, for example, are certain countries particularly religious or militant or magically inclined?
Slowly you can start to draw a sketch of your world by hand, where the countries, if necessary the sea and possibly their names or artificial names ("the magic elf city") are on it
Sketch of Euphari’s Map
This is an initial sketch of the world map of my second campaign, I tried to capture the main elements of the countries, like "Elven realm magical" or "Human kingdom belligerent" and partly already thought about what could be important rulers or symbols of the countries. Also especially important places, like here the monastery or short information about areas "big forest, many animals" you should note here. You can keep the outline very simple and write everything down later. I also thought about some river courses and islands, but they changed later on. Generally your sketch should not be anything fixed yet, but still very variable. Nothing is set in stone up to this point.
Relationships of the countries
After you have already thought about possible conflicts and alliances at the beginning, you should deepen them further. From which past events did the conflict arise. How exactly do the individual countries relate to their neighboring countries or to the parties of a (possible) war?? Here you can also consider trade relations between the countries and, if necessary, the sea. write down traded goods
Create people groups
Together with the general mood of a country it is very important to describe essential people of the countries. This includes important guilds of the countries, such as trade guilds, mage guilds or even criminal guilds, as well as important political / economic circles, such as often the royal house and close followers or, for example, the royal council. These groups of people usually represent the moral ideas of a country or are just there to eliminate them. Again, conflict between factions often adds spice to the story and causes players to take sides. Do you prefer to get on well with the local mage guild or the capable craft guild from next door? How exactly you can create NPCs you can find here
Create Map
One of the most important elements of a GM is the map of his world, of course this can only be there as a sketch and therefore only for the GM. But from experience, it really helps and pleases players to see where they are in the world, where they have been, and where there may be sections of the world yet to be traveled. Thus, I highly recommend creating a map that serves you and your players.
For this purpose a website for creating maps is a good choice. For example, I have been using Incarnate for over a year now. This is a tool for a small price per year, with which you can easily create medieval maps, city maps and partially dungeon maps (dungeon maps are just not so good with the tool). Almost all the cards shown here are designed with the tool. Of course you can also draw your map yourself, even if you are not particularly talented in drawing, they can look good. Here are my example maps for my two campaigns, self-drawn and with the Incarnate Tool.
Aldrien Map self-drawn
Aldrien Map with Incarnate
Eupharis Map with Incarnate
For the map, remember that countries should have a natural border with their neighboring countries. These can include a river, a mountain range, a volcano, a forest, a magical construct, or the like. So always draw a natural line between countries. For the coastlines of the continent I would strongly recommend to look for a nice one at Gmaps, maybe at the beach of Mexico and use it for your map. So the card looks much nicer than with a simple line. Then you can think about the size of the countries depending on their military power and origin. When you have finished designing all countries, you can either color them to distinguish them from each other or design them with colors and symbols depending on their ecological background.
History of the countries
You can consider the past of the countries in addition to the present state of the countries. Past wars, conflicts and emergences of socialization are particularly exciting topics in history. Which country developed first? With which other was it at war? Do you still notice remnants of it? Were there any particularly important historical figures, such as the leader of the warrior nation who conquered one land after another with a powerful but obsessive sword and could only be stopped by an alliance of elves, dwarves and humans?. This offers then for example directly a hanger for the Quest around the cursed sword.
Create cities
Now that you’ve created and thought through your countries, it’s on to creating the big, important cities of the countries. I have designed a capital city for each country and defined a few large cities in special countries. You should decide how detailed you want to make your cities. You can also make maps for individual cities here, but I would only recommend this for really important cities, or if you really enjoy it, because this can be a lot of effort. However, it also becomes less per time that one makes it. You should come up with the city’s attitude to life, roughly outline the local ruler and nobility, and define the relationship between the nobility and the city’s inhabitants. Which peoples live mainly in this city? How is the wealth of the city distributed, are there many very rich and many very poor? In addition, you should roughly note important buildings in the city that may be interesting for your player characters to visit later on. It can be the colloseum in the warring human city, the overflowing library in the distant city, or even the highly frequented warm springs. Just write down the locations as keywords, you can also consider them when creating the city map.
Create a map of the city
Again, you will be faced with the choice of a self-drawn map or an incarnate map (o.a.). You can create the self-drawn in Paint or in any drawing program for better readability, giving the rough complex names and roughly drawing the outline of the city.
People city with drawing tool& Graphic tablet outline