Photoshop Face Swapping.

1. Choose your images.

2. Switch to the Quick Selection Tool. (Pic 1)
3. Select the face using this tool.(Pic 2)

1. Copy and Paste the face onto the other image. (Pic 1)
2. Drag the face into the right position and use free transform to fit it. (Pic 2)

1. Grab the erase tool, use 0% hardness. (Pic 1)
2. Erase the parts that aren't needed. (Pic 2)

1. Go to Layer>New Adjustment Layer>Levels (Pic 1)
2. Make sure you check "Use previous layer to Create Clipping Mask" (Pic 2)

1. Using the levels, make the skin tone as similar as possible. Sometimes more editing is required. (Pic 1 + 2)

Now you're done, and sorta creeped out...