Hi Ania,
In which direction would you like the cill to slant? I'm guessing you mean you want lower part of the window to follow an angle (perhaps over a roof) leaving you with a non square window.
Two ways to model this - either build a custom window using slabs/walls, (see the section on 'Graphic Creation of Custom Objects' in the reference guide for instructions.)
Or use the freeform window mentioned on this thread -
. This is the easiest option.
Also if you wanted a particularly complex cill, use a separate complex profile assigned to a sloping beam, and place in front of the window manually.
Hope that helps.
