So, I've so far been just copying half of a players face (depending on angle, maybe more) and duplicating, flipping and merging since most reference pictures have light differences. Obviously if someone has a particularly asymmetric face this may be tricky.
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Copy it into the template (I am using a hitterman-esque paint tool to line up eyes etc, rather than the UV map).
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Line it up and use warp to move features in line.
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Duplicate, flip, align and visualise the horror.
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Merge, dluplicate and do any touch ups you might like.
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Duplicate the background and adjust brightness etc to roughly match skin tone. CTRL+left click the picture on the layer of the face to select the face.
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delete selection from duplicated background layer. highlight modified BG layer AND face layer. edit > autoblend layers > OK
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merge the layers and do any touchups with clone stamp etc. save as JPG.
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import and fanny around with the face gen as you normally would to get it looking better (didn't do anything but apply the face here, still needs playing with).
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I'm sure I'll have cocked up the picture order somewhere but hope that helps.