1. Open up your image in Photoshop and duplicate the layer (shortcut key: cmd + J)
2. Select a new adjustment layer, and from the menu select Hue/Saturation. Turn the Saturation down to -100
3. Select your image layer (the layer you originally duplicated at the beginning of the tutorial) and change the blend mode to colour dodge. Invert this layer (cmd + I) so that the layer looks almost or completely white.
4. Go to Filter > Blur > Gaussian Blur. Slide the radius until the image looks correct to you
5. If parts of the image look too dense, select your brush tool and choose a large brush size, with a hardness of 0% and an opacity of 40%. Make sure your foreground colour is black, then paint over the areas that are too dense.