I'm a big fan of using maps, particularly world maps, as an early part of preparing a world for an RPG. If the game comes with a pre-packed setting then it has likely already done all the hard work for you on world building. All you have to do is learn it. However, when custom building a world you have a lot of options on how to approach things. My preference is to do a broad overview so I have a sense of the whole play area, and then to hyper focus on a small section of that world where the game will be starting.
I like this for a couple reasons but the big one is I plant seeds for future stories, and I can give my players a glimpse of the whole world before we just focus on the world around their characters. This lets players pick where they're from - at which point they can help you build that world. It also helps cement the feeling that there is a whole world out there. After all, they can see it.
The challenge this presents though is knowing there is a whole world means you also have to build it. Only world building a whole world can take a lot of time. If you have that time, that's fine. If not? Well, there is a reason the "lets get ready quick" advice is all about only prepping what you need and nothing else.
Where's the middle ground? It's in brief overviews of the places. Enough to give the flavor of what is there. A structure to hang a tarp over. It suggests what is there. Gives enough form to work with if you have to improv. Ultimately though it's just a tarp over an area still under construction. Part of hanging that tarp is having quick builds. So how do you quick build a kingdom? Easy. Just define the following things:
1. The Name of the Kingdom
The kingdom needs a name. There are plenty of generators out there if you google search "Kingdom Name Generator." Or you can make one up yourself. Remember, places are frequently named after people, environment, or historical events and countries are no different. Give your place a name. The story of what the kingdom is can begin here.
2. Type of Government
Is your Kingdom even a kingdom? Is it instead an Empire? A republic? A democracy? What type of government does your place have? The default answer in Fantasy games is for various forms of monarchy, but even in the confines of "monarchy" you have a lot of room for variance from a straight feudalism set up, to a monarch over an oligarchic parliament of some kind, to a monarch over a democratically elected senate, or all sorts of other variations. How the place is run is important if you ever have to play in the area or someone from the area. Especially if dealing with members of the social elite castes.
3. Who Is The Current Ruler?
You have your government, so who is perceived as the current ruler or person in charge? The United States is a democratic republic, but our 'person in charge' is the leader of the Executive Branch a.k.a. the President of the United States. Other types of governments have other titles for this, but be it a figure head or a wielder of true power. Name a person in charge - and here or in type of government give yourself a note if they have 'real' power or are more a figurehead.
4. Type of Currency
If you are doing a fantasy universe and just doing "Gold Coins" or "Silver Coins" that's fine. However, you can add a lot of flavor to a country with their currency. What a kingdom calls their money can tell you about it. There is a reason a lot of old currencies and fantasy currencies have names like "Crowns" or "Royals" for their coins. You can also decide what or who is on the coins. Monarchs are popular figures for sure, but there is also a big difference between a country that puts weaponry on their currency versus one that puts books or religious symbols.
5. What Are They Known For?
What is the country known for? What is their claim to fame? Sparta, or so the legend goes, was known for their military prowess. Athens for their democracy and philosophy. Rome was known for their expansionism, among other things. Does the country have a chief export? Do they have a chief import? It doesn't have to be a commercial good, but countries work together as a type of network. Everyone gives and takes - but not necessarily in equal value - so what does this country give, and what does it take?
6. What Stereotypes Exist About The People Of The Country?
Finally, what are the stereotypical views about the people from that country? The U.S. has a reputation for being cowboys, gun crazy, and obese right now. Canadians are 'known' to be nice and always apologizing. The French have a reputation - at least in the U.S. - of being kinda assholes with a 'better than you' attitude. The Irish are 'known' to be drunks. And so on.
In my experience most of these stereotypes are negative, but some good ones will also be there. But what are the widespread views of the people from this country and are they accurate? If so, for a small group or everyone? Remember, many of these stereo types will come from enemies more than allies. Those who like us tend to know us as people. It's those who dislike us, or don't know us, that tend to generalizations.
Bonus: Allies and Enemies
As a bonus, who are the country's allies and enemies? You don't need why, generally the reasons are myriad or just "because." This isn't as necessary as the others for using the world from just the frame, but it is good to know. Especially for a more fighty type country.