These windows I choose to install casings and sills. Since the actual window does not have a sill and is really not designed for having one I had to come up with a way to have both visual features and provide support. Every situation is different so each one will most likely require modified designes however, wraping a window is pretty much straight forward in any case.

Here we have a set of windows made from pine with prairy grids up and down. They are stained a little darker than I originally wanted with the risk of not matching the oak window casing I will be installing.

This is a piece of MDF that I used to mock up the dimentions for the groove and biscuit setup on the sills. The 3/4" wide 3/4" deep groove is to overlap the existing bottom face rail to give the sill an even look.(also provides more gluing surface). The thickness of the overlap is 3/16" thick.

Just a shot of the groove and the layout lines for the notch cut.

Here I started cutting the biscuits to meet the ajoining cuts in the window frame. Marking and set up is very cridical here. But if you mess up just re-estiblish your marks for the biscuit joiner and make your adjustments. You won't see the mess up when the sill is in place.