I made triangular pieces of wood and placed them next to the windows. Then I installed window sills on top of those triangular supporting pieces - which makes the sills to be 15 degrees tilted so that rainwater will flow down the sills. I almost forgot to take a picture, so this is when the actual work is already done and I was packing all the tools - and then noticed that the leftover pieces of timber would make a picture of the day.