Okay, I'm not sure if I can explain this properly, especially since I just mess around with each individual fruit pic until it looks right - there's no one way of doing it. (Here are some.) But this is pretty much how it usually goes:
First, open your fruit picture. For this tutorial I've chosen an orange.
Then open your face picture. Try to choose one that is in similar lighting conditions (e.g. if it's side-lighting, natural light, artificial light, etc.) The orange is lit with studio lighting so I've chosen a studio shot of Noah.
You'll notice that for the orange the light source is to the right, and the same for Noah. If this was not the case, I would flip the orange horizontally so that the light was coming from the same direction for both images.
Use your lassoo tool to select and copy the face portion of the person and paste it onto the orange. This will create a new layer.
Drop the opacity of the face layer so that you can see the orange through it - take it down to about 70%. Then you can move and resize the face until it sits correctly on the orange.
Increase the opacity to 100%. Use a soft brush eraser to erase all around the features.
Now duplicate your face layer. Hide one and work on the other. I run my Simply Black-and-Whtie Contrast action to create a stark black and white. You can do this any way you like, but it will require more contrast than simply desaturating it. Give it some contrast with Brightness/Contrast, Levels or Curves.
Now go to Image - Adjust - Variations and give it some colour until it looks like it pretty much matches the fruit. Adjust with Colour Balance to get it right.
Okay, now is where it takes a little playing around. First, drop the opacity of the face to about 60%. You can now see the character of the fruit skin showing through a little. But you lose some definition in the shadows around the nose, etc. So you need to boost contrast on the face again, until the definition comes back.
Use a soft brush eraser on low opacity (about 20%) to start to rub along the cheeks, bridge of nose, etc to let the skin of the fruit show through. If you notice that the fruit is darker than where you've erased, then dodge the fruit to match the highlights you've erased (this creates contours on the actual fruit skin.) If the fruit skin is lighter than it should be, then burn it.
Now go to the fruit layer and burn the shadows. Don't be shy - this helps all the little pores and textures to show up better through your face layer. Keep working at it until it looks natural. You need to start messing around now with increasing opacity of the face, working on the fruit and erasing a little more on the face, etc, until it starts to look natural (ha!)
Go back to your face layer and burn the shadows of the eyes, nose and mouth so that they are "there" enough - especially the eyes.
Okay now duplicate your fruit layer and merge it with your face layer. Remember the original colour version of the face layer, which you've made invisible? Make it visible and drag it beneath the fruit-face layer. Now is the fun part! Zoom in and use your eraser at 100% opacity, using various brushes, to erase the inside of the mouth and eyes so the colour version shows through.
Voila, your fruit face is complete!
