If you use wwWidgets, you just need to build wwWidgets, put CONFIG+=wwwidgets in your .pro file, and you're ready to go.
Not entirely.
You also need to provide the location of the lib and its headers.
Then, its exactly the same as in Qt qmake/pro files.

Now I seek advice about :
- when I build my output, where should the output go (the binaries). Not just anywhere among the Qt binaries I suppose.
Where ever you want.
Usually people put binaries under ./debug or ./release, *.obj under ./obj etc...
But its fully up to you.

- should I copy an include file for my stuff, or just have the user include the 'include' directory of my project ?
What kind of user?
A user that runs you executable - then no, he only needs to run the exe. Your setup should take care of putting the libs and dependencies where the exe can find them.
A user for a lib you are writing, will have to get all the symbols files and header at least, and code if its not closed source.


- what to put in this .prf file ?
- where to put this .prf file ?
http://doc.qt.nokia.com/latest/qmake...ation-features