I still don't understand the backend

[Rune Zedeler]

I am sorry, but I still don't understand how the backend works.
The documentation is very hard to understand when I don't understand the
basics of lily.
I want to squeeze some of my music together horisontally.
I guess that I have to reduce the value of arithmetic_basisspace of the
SpacingSpanner - but I still don't understand HOW to do it.
How do I find out where I find that SpacingSpanner - is it part of the
score, of the staff, of what?

-Rune