First you need a map. And a compass. The map is a large sheet of paper with a representation of what 's on the ground printed on it. The compass has a little needle that point north.
But there's this great new invention called GLobal Positioning System (GPS). It's like a real map on a tiny computer. You know what a computer is right?
AVC: So where are we?
MacMillan: It looks like we're standing in a giant field of mushrooms
AVC: Well if you look at the bottom of the map it will tell you what those musrooms depict.
MacMillan: Oh, it says mine. So these mushrooms must belong to whoever drew the map.
AVC: No, I think you'll find we're in a minefield
MacMillan: So what do we do if we step on a mine?
AVC: Normally, you jump 100 feet into the air, and scatter yourself over a large area.
But seriously, game mapping takes a long time, especially for FPS games. You have to construct the entire map in 3D, like walls, doorways, other structures. Some stuff like vehicles can be found ready made. I spent several months building a Half Life map. There are normally lots of online resources for ready made objects like furniture, or lifts (my map had a working lift), cars, fridges, (I also had a working microwave that blew up after one cycle, and a soda machine). But most of the map is hard work.
Then you have to select spawn points, textures, lighting, sky effects, etc. You will also need a decent computer to compile the map, which can take hours depending on the complexity.