how does one go about creating a mod for an existing game?

I have a demo of unreal tournament 2003 and because i am such a cheap skate rather than buying the full version i’m planning on modding the demo (make new maps, characters and guns)
does anyone know where exactly i should start?

I dont see what this has to do with blender exactly, but I know there’s an unreal editor, that comes with a full version of the game, and normally you would use that to mod for it.

UT 2003 should be in the bargain bin by now.

I don’t see why you can’t just buy the game.

If I remember correctly, you cannot mod the demo.

You can’t mod demos. The full version usually comes with the needed tools.

And yes UT 2003 should be very cheap now, UT 2004 costs 15€ here. Considering the time you will spend with modding, your best bet is to buy the game.