Here's the original picture.

I started with a light rotation of the whole picture, and I started with the horns by selecting & copy/paste the left leg of the turtle. I made a polar coordinates distort to it, rectangular to polar. I moved it and rotated it to put it at the right place, and adjuting the shape with a layer mask. Layer masks are important to give the flexibility for taking off and putting back later some details. The right horn is a duplicate of the left one... flip & rotate...

I made a copy/paste of the left arm & shadow and moved it at the right place to built the left side (bottom) of the face.

Now, with the layer mask, I kept only the details I needed. Always using the brush tool on a layer mask, adjusting constantly the size, hardness & opacity, depending of the detail to do. Taking off & putting back details by switching foreground & background colors (for layer mask: black & white) by pressing "x" on the keyboard.

I started to make the shape of the right side (top) of his face only by stamping the background over some parts of the turtle on another layer. The chin was a copy/paste of the head of the turtle, I made a shear distort & free transform. After, always the same thing... moving, rotating, taking off some parts using the layer mask...

Copy/paste of one part of the shadow... moving & rotating, layer mask...

Now I used the left arm for the left ear... copy/paste, free transform, layer mask...

I made the details of the ear with the stamp tool & brush tool on different layers. Can use different blending modes on layers with the brush tool for shadows & highlights. I was gradually cleaning & adjusting some details with the same tools.

I made the new shapes of the shadow using the pen tool, after I used the stamp tool to clone some parts of the original shadow, adjusting & cleaning with the brush tool, and adding a gaussian blur.

Copy/paste a detail & moved it up... this detail will be part of the devil's mouth.

With the stamp tool, I took off all the details that I didn't want, preparing the face of the devil.

Copy/paste & moved some details & highlight of the turtle for the left eye & the nose.

After that, I draw some details with the brush tool & stamp tool, adjusting the size, hardness & opacity of those tools as usual.

I copy/paste the left eye to make the right eye, move-flip-rotate-free transform & layer mask to keep only a part of it.

Here, I added some red reflexion to the right eye & cheek.

I added some small details again, and added a curves adjustment layer for a better contrast.

Finally, I was close to forget the right ear... copy/paste of the left one, flip-rotate.......

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