Hello and thank you for your interest in modding OXC!
To make a UFO, you will need a new map (unless you want to reuse some other map, vanilla or not, but I assume no). This is a fairly comprehensive subject, but you can find specific info here:
https://openxcom.org/forum/index.php?topic=7664.0Once you have the map (and a routes file for it), all that remains is to:
- create a new entry under UFOs,
- create a new entry under alienDeployments,
- do something for the UFO to actually appear in the game (for example, by adding it to some mission).
There is also companion stuff like name, Ufopaedia etc.
For a working example, I can recommend my veeeeeery old mod:
https://openxcom.org/forum/index.php?topic=2144.msg21155 - it should be fairly simple to analyze and copy. Feel free to ask questions.