It is possible to download and play predesigned maps, which could be in this menu, as well as making your own map. The C-evo website offers some that people have created.
It is easy to play with random maps: when the game is started, the computer makes a map that has water tiles and islands and usually continents, generally with a strip of fairly cold-looking Arctic or Antarctic terrain at top and bottom. Each game has a different-looking map; no two games are the same as a result. Every game allows you to explore a new world. Despite the randomness, you can select from menus the map size and the percentage of land.
An important option is the size of the map; you may choose one of six sizes of random maps, labelled 35%, 50%, 70%, 100%, 150%, or 230%. Premade maps can have other sizes.
On the three smaller sizes, if you have many opponents you will probably find that one or two of them start so close to your capital that you cannot build even a second city in a really satisfactory location. Choosing a high land percentage alleviates that problem a little, because there is more land to share out. For example, the 70% size with 70% land generally gives you a couple of new cities on good sites before the nearest rivals' territories expand so as to be really restrictive.
Land v waterEdit
You can choose the percentage of land on the map, in 5% increments from 10% to 100%.
At about 50%, you start to find that the ocean is cut up by continents so that there may be two or more large seas and no way to sail from one to the other. Look for a narrow isthmus where you may build a city to link two areas of water or (after you discover Explosives) build canals.
With 100% land, there is no value in researching Navigation. However, there is still value in researching Map Making early because, in addition to letting you build Longboats in ports (which you don't have), it adds a trade point to every river tile you work.
Whether you choose a random map or a pre-made one, you then have an option to edit it. You can change any tile and can (actually, you must) specify starting positions for capital cities, including one for you to start on - or two or more favored sites, letting the computer decide which one it will start you on.
The resulting map can be saved with a distinctive name so that you can use it again and again, with or without further modifications. However, if you have chosen a pre-made map, you may need to open it in a new window before saving any variation of it, or you may lose the original. As a precaution, you could find the map folder (possibly in a location with a name like C:\users\robinp\AppData\Roaming\C-evo) and make copies of all before doing any edits.
Starting the gameEdit
After you have highlighted a map, whether or not you have edited it, click on the "New Book" tab to start a game with that map.