UPVC Window in Vertical Tiled Wall

[jackofalltrades]

I need to install some UPVC windows in my extension, which will be tile hung to match the existing house. I think I need to frame the window aperture with a UPVC capping board or similar, to tile up to. Should I fix this first and fit window over the top of capping, or fit window and then capping board? Obviously the first way results in a smaller window.
Many Thanks.

[welsh brickie]

fit the window,then cap after you tile