Fitting a wooden or plastic window sill onto plaster sill

I personally prefer to use the pre primed mdf window sills they give a cleaner finish if fitted correctly. Cut the sill for the 50mm longer than the opening it can be cut straight, no mitred cuts are necessary. The sill will extend on the sides and the front of the window opening 25mm (1 inch). When you cut the wood, cut notches on the sides so that it sits within the frame. You will essentially cut a "T" shape into the sill. If stuck look on youtube or other site on how to fit a wooden window sill. Thank You C&B CONTRACTORS

I regularly use a plastic l shaped fascia board for window sills. Just cut to size and glue it with gripfill/no nails etc. Silicon to the window and around the edges. Gives a lovely finish against pvc, easy to fit and easy to clean.