Well, what can I say about this one? Extremely tricky to paint. Several times it was nearly (yet) another wiper.
The method is most accurately described as "winging it", but broadly I began with an underpainting of dilute raw umber and blocked in all the really dark darks. I didn't do anything about the windows or the reflections at this stage.
When that was dry (overnight) I painted the darkest half of the painting which is roughly to the left of a diagonal line from bottom left to top right. I also did the cat's face but not his tail, fronts of his legs or his whiskers. Then, using a ruler, I painted in the black bars in the window glass.
That went very badly. But I left it alone, all the wobbly bits and everything, and waited for it to dry.
Next day, I thought - those window bars are rubbish. So I drew over them using a Sharpie. Honestly, I do not know how to paint lines with a brush. Just as well I am not attempting to paint ship's rigging or similar.
I did all the squares of colour in the windows (this also covered up any wobbles or blobs in the black lines) and then finished it all off with the highlights on the cat and his tail and window glass reflections. The latter were just hinted at: the result was better without too much about precision.
Then, when it was properly dry, I glazed over the dark half with transparent oxide red to warm it all up a bit.
Since which time I have been recovering by eating too many mince pies.