It’s a good idea to work with a face that’s larger than the face you’re trying to swap out. It’s easier to fit a large face over a small one. When you increase resolution in Photoshop or Lightroom, you can create a higher-resolution face, so 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.