One morning I took a photo of the strong sun on my flowerpot on my windowsill. I like the intense highlights on it. I decided I did not want the entire window, just the flowerpot.
So I cropped the image until I had what I wanted to focus on.
I used chalk and sketched in the placement of the pot and windowsill.
I used my technique for painting glass. Paint the background as usual - here the sky is a darker value at the top and lighter towards the bottom. I added some shadow area that will be cast by the pot. But mostly ignored the pot and flowers at this point. I used a softer brush to get a smooth finish. Glass needs to be smooth for the best illusion.
Then to make it look like glass, I streaked some very thinned white paint vertically to create the illusion of glass. Don't get heavy handed or else you wind up with birdy doo-doo. That's not good. You can use more or fewer streaks to get the look that you are after. The more streaks the dirtier the glass will appear.
Demo will continue next week.