Step 1: Find the right pictureMake sure the 2 faces are roughly facing the same way.
Step 2: Resize the FaceCrop the face
Resize and tilt it until it fits the body. (Edit -> Free Transform)
Step 3: Adjust the ColourAdjust the colour balance (Image -> Adjustments -> Color Balance...)
Also adjust the level (Image -> Adjustments -> Levels...) and
brightness & contrast (Image -> Adjustments -> Brightness/Contrast...) to fit the body.
To me this is the hardest, cos I'm quite weak in figuring out the best colour composition.
Step 3: Amend the BodyUse the King-of-Tools in 'Photoshop Plastic Surgery' ====> Liquify (Filter -> Liquify...)
Notice I also cut off the 公鸡头

And
Voila! There you have it.
And as mention above, because I'm quite weak in colour adjustments, I prefer to have it in Black & White. So as to cover those imperfect blend. =)

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This is how I would look like in 6-pack. Muahaha.