Method:
Saw picture and had idea
Booked model for photoshoot
Did shoot which was not quite what she anticipated
Left remains of model next to stream
Or:
Found previously taken picture of model laying upside down in chair
Re-angled her legs so they looks as if they were straight rather than pointing skywards
overlaid her in picture and added shadow
Cut throat using photoshop brush
Added blood to water using low opacity hue
added more hair
used another layer to darken the area using low opacity black brush
removed sign from bridge as it drew the eye
played with levels, sharpen and blur until satisfied