Thank you pent.

Your explanation is very helpful.
While I was trying a few questions arose.
Can I open multiple images? My window texture is 15m by 11m. How would I accurately represent this in my UV Map? Scaling/Resizing can't be done very "cleanly".

I wanted it to be exactly 15m by 11m.
(In other words, instead of positioning it and sizing it by mouse cursor, can it be done through text? Say define the scale, and the position by numbers?)
Smart UV Project or just UV Unwrap?
After being satisfied with the look, how do I get my UV Map textures to my model? If I export it, the textures don't come along.

Sorry for so many questions. Thanks again
