According to Do It Yourself, there are five steps involved in fixing a window sill. These steps include measuring, accessing the sill, cutting a new window sill and fitting the window sill. With the right tools, window sills can be replaced in an afternoon, according to How Stuff Works.
The first thing you need to do before repairing a window sill is to make sure you measure the length, depth and width of the window sill, so you can select an appropriate piece of wood and cut to size. Once you have all the dimensions, the next step involves trimming the sash channels and the casing trim using a pry bar.
You should rest the pry bar on a piece of wood before you exert any force to avoid damaging the window sill. After removing the trim, pull out all the nails using a pair of pliers. You can then cut out the shape of the window sill on a new piece of wood with a saver saw, and sand down the edges when finished. To refit the trim, replace it in the reverse order from which you removed it, and use a couple of nails to hold each part until the fit is good.