[Delevingne]

Use a brush with hardness 0% (not a pencil)

[RozaytaHF]

One thing you can do is add noise then blur it. Filter>Noise>Then add some noise and use hue and saturation on it until it looks good. Then you could use a small gaussian blur and expand the image out and cut it to a square and such to make the fades go away. Only thing I can think of off the top of my head.