You take your laptop full of girlie pictures to your dentist, and ask them to stick your laptop in their dental X-ray machine.
Just kidding. In a paint program, it usually entails these steps:
Separate foreground from background by using the magnetic lasso tool to create a new layer.
Use the smart fill tool to patch up the area that was behind the person
Create a layer mask from this patched area, and arrange it on top of the layer of the person. - this should leave a visible and seemingly invisible portion of the subject.
Use various brushes to create a fading effect around the edges, to make the visible to invisible effect more gradual.