This is a sort of subquery to the question of how to change window
managers.
So the question is: supposing you compile a window manager yourself,
so that it does not come from the packaging system.
What is the standard best way of setting this new window manager as yours?
It certainly won't exist in any of the lists on the login page, since
debian could not know about it.
So, i googled around, and there's this page
http://www-rohan.sdsu.edu/doc/debian/ch-X.html
which recommends just making an executable file
/home/USER/.xsession
and putting in a sequence of commands there---which
could just be the path to your window manager.
I tried this, and . . . . it works.
But i feel a little uneasy about this, like this is not a best practice,
or some newer release of debian may break it.