Is there a tutorial or something about it to get me started?

I'm a complete newbie and I would like to start with something simple. Could you point me in the right direction here?

I'm trying to add a window decal like this on the Blista compact windshield:

By the way that cool car is the 1980 Honda Prelude by Ghost0912: 1980 Honda Prelude 1.1.

I poked around the Blista's texture files but I didn't find one containing the windows texture, where is it and how can I modify it? Thanks!