We have to create a new layer and group it with that one the ornaments are drawn on: Layer ? Create Clipping Mask (ctrl+g). Then select the Gradient Tool with the next shown adjustments to paint the new layer:



Create one more layer, group it with the previous one (ctrl+g) and change the layer's Blending Mode on Linear Dodge. Select the Gradient Tool with the previous applied adjustments and paint the layer, but this time the painting should be done on the opposite direction.



One more layer, grouped out. Apply on it the next filter indicated here: Filter ? Render ? Clouds, choosing the black and white colors.



Change the layer's Blending Mode on Screen: