Hey Tilli,
In your /base/ folder you've probably got a series of .pk3 files. I think there's one called 0ufos.pk3 and you need to unpack it. You can use
7-zip to do so. It will unpack into a /ufos/ folder and you'll see tons of .ufo files inside, which is what UFOAI uses to configure a number of things. Depending on how you get the game, you've either got the folders or the pk3 files, which just pack it up to be a bit smaller.
There you'll find the maps.ufo script. You should take a look at the maps.ufo script just to see how a map is defined. But if it's just a test map, I'd recommend making your own .ufo file. It's just a basic text file (use Notepad or Notepad++ if you have it) with the following code:
mapdef yourtestmap
{
map "yourtestmap"
description "_A test map."
multiplayer false
maxaliens 8
aircraft
{
// Just for your test map, leave this empty
}
ufos
{
// Just for your test map, leave this empty
}
terrains
{
// Just for your test map, leave this empty
}
cultures
{
// Just for your test map, leave this empty
}
populations
{
// Just for your test map, leave this empty
}
}
Save it as "yourtestmap.ufo" in the /ufos/ folder (if you want, you don't even need to unpack 0ufos.pk3, you can make your own folder and drop it in). This will make it appear in the skirmish menu.