Michael Orton originally developed the effect named after him by sandwiching 2 slides together. Both were over-exposed and one was deliberatey out of focus in order to create an effect like a painting. It was hit & miss, and also time consuming to create, requiring intention and time to get right.
In the digital age it's still possible to work that way, but by the miracle that is modern technology we can also use a single RAW file direct from camera, modifying it to suit. This is the unprocessed image saved as a .jpg file.
I tend to think of the 2 layers required as the 'detail' layer and the 'colour' layer.
The detail layer needs to be over-exposed to replicate the 'thinness of that original transparency, but also sharp and contrasty to create the outlines that one might find with a watercolour. To create this I upped exposure about 1.3 stops, raised contrast and especially clarity, reduced saturation and vibrance.
The colour layer doesn't really require much detail, but I also didn't want something that was horribly smeary like a child's painting. To create this I worked contrast & clarity in the opposite direction, pus reduced saturation & raised exposure again so that this would sit below the detail layer and just provide soft colours.
And finally they required blending. I used On1 suite Layers program, blending with a multiply setting in order to simulate sandwiching 2 transparencies, with the top detail layer set to roughly 50%.
To me, that looks quite like a painting, yet holds a reasonable amount of detail and balances the colours out nicely in the mid-range in a way that an ordinary photo wouldn't. There's a lot of learning and practice to be had yet, but this is a technique I may use more in the future.