Place your start photo on a layer above a solid black background, desaturate it with Image>Adjustments>Desaturate and invert it with Image>Invert. Use the photo as a mask for itself by going to Select>All and Edit>Copy.
Add the layer mask and then paste onto the mask file. Set the layer’s blending mode to Linear Dodge and reduce the Opacity to around 35%. Colourise the layer with Hue/Saturation if you wish to add the blue or green hue often associated with X-ray imagery. Touch up the mask using a black brush to further conceal details you don’t wish to be visible.
Next bring in the skeleton imagery and align it as needed beneath the photo layer. If you’re unable to find a matching skeleton image, consider using a 3D model for the solution. Invert the skeleton layer and set the blending mode to Linear Dodge at 50% Opacity. Colour as needed and use a layer mask to conceal areas like the eye sockets that should not be visible.