It’s a good idea to work with a face that’s larger than the face you’re trying to swap. It’s easier to fit a large face over a small one. Also, with a larger, higher-resolution face you won’t have to deal with distortions from pixelation as a result of resizing.
Drag the face you’re swapping onto the head you’re using and then gradually resize and line up the features. Turn down the opacity in the face layer so you can see the original features underneath the face you’re using. Match the eyes, nose and mouth and gradually move the new face over the original. Once you have it placed to your liking, adjust the opacity back to 100%. The new face should be placed as naturally as possible over the old one.
Blending and colour-correcting a face swap.
The lighting and skin tones of people’s faces can vary wildly and sometimes one person’s look can change across different photos. You’ll almost certainly have to correct for subtle differences between the two photos if you want your face swap to look professional.
Match the multiple skin tones with the Match Colour feature (Image › Adjustments › Match Colour), using whichever image you want to match colours from. Gradually adjust colours to match the skin tones together. Use the Blur tool to match the amount of blur on the head and face and use Auto-Blend Layers with Seamless Tones and Colours selected to merge the face and body layer together.