There is normally quite a large gap between any window and the surrounding masonry! This is often covered by means of a wood window lining, which covers the gaps and is normally finished with decorative wood, such as architrave. Today this gap, known as a reveal is often covered with plasterboard and gives the impression of a larger room. Wood can be used to make the gap between the masonry and the window small enough for one piece of plasterboard but here we have chosen a method which also uses up spare pieces of plasterboard.
Here you can see that there is approximately a five inch gap between the masonry and where the window lining should be. Rather than try to use large blobs of dry wall adhesive and one piece of plasterboard, we are using 5 pieces of plasterboard and gluing each piece to the previous piece using dry wall adhesive, as to build the thickness up gradually.
Apply some dry wall adhesive to the masonry and then push a piece of plasterboard into the adhesive. Apply some more dry wall adhesive and then another piece of board.
Repeat this process until the plasterboard is almost level with where you wish the window lining to end.