said by Dr Bombay:
what? couldn't be easier:
open image.
filter > artistic > cutout.
click ok.
Again, you must consider the contrast and shades of the original image. It isn't as simple as click, click, click, click.
There are some settings in the Cutout filter that can be changed to improve the results on marginal images. Let's don't blow through that suggestion so incompletely. There will be images that don't provide good results no matter what values you use in the filter.