Fitting an interior window cill

[Tigger90]

I have solid granite and brick walls and need to fit the wooden window cill before the plasterers arrive. Do I chase out a small portion of the window reveals and slide the cill in ? Would solid oak be a suitable wood for the cill?
Thanks in advance

 

[gregers]

if i understand this correct your getting plasterers in?so no just fit the window board and let them plaster it in.
oak would look really nice but it may move a bit if it gets wet from the plaster???