The Process:

Step One:
Prepare a wash of equal parts Potassium Ferricyanide and Ferric Ammonium Citrate mixed in distilled water solution (concentrations vary)

Step Two:
Apply wash to surface of material to be coated, paper, cloth, porcelain etc, and let dry for one hour+

Step Three:
Insert photosensitized paper into contact frame with negative lying emulsion side down on its surface

Step Four:
Expose print for 5-10 minutes, or longer given indirect sunlight

Step Five:
Remove print from frame and rinse for 5 minutes in running water

Step Six:
Wash print with hydrogen peroxide 3% to fully oxidize to deep Prussian Blue

The Science Step By Step:

Step One:
A light sensitive material is made more sensitive by the addition of another agent, of independently low sensetivity.

Step Two:
These chemicals are absorbed into the surface or fibers of a material to be dyed photochemically.

Step Three:
Once water has evaporated, crystals of light sensitive chemicals remain bonded to print medium. This sensitive paper may be stored for prolonged periods before exposure if a consistent darkness, temperature, and humidity are maintained.

Step Four:
The action of sunlight onto the photomedium produces a deep blue colored chemical known as Prussian Blue, exposures to direct sunlight are on the order of 5 to 10 minutes.
The further action of this light chemically reduces this Prussian Blue to Prussian White, allowing for more light to penetrate deeper within the medium, allowing for effective increased exposure. This vastly shortens the time needed to produce an image. This is seen in the whitening of the image following its change from an already light yellow-green to deep blue.

Step Five:
Washing the exposed print in water rinses out the water soluble light sensetive chemicals the process began with. This leaves behind the insoluble Prussian Blue and Prussian White, which oxidize in the water to reform a deeper Prussian Blue image.

Step Six:
Hydrogen Peroxide provides oxygen for the complete and rapid oxidation of the print to its maximum Prussian Blue density. 

Preparing an internegative via contact printing from an enlarged interpositive.

« To the natural philosopher there is no natural object unimportant or trifling. From the least of nature's works he may learn the greatest lessons. » 

- John Frederick William Herschel (English Polymath 1642-1726)

Sensitized unexposed paper,
exposed and unwashed paper (note gray/brown/purple "...Violet ruddiness..." from Prussian White formation),
final print washed and drying.
(Left to Right/Top to Bottom)

These images show the progression of Prussian Blue and White formation during the exposure of a print to direct sunlight. From the first image to the second, a pale yellow-green color is noted to deepen to a blue during the early formation of Prussian Blue. Following further exposure to light this Prussian Blue is broken down to a colorless Prussian White that returns to blue upon its oxidation in the final washing phase, this is seen as the print lightens to a paler color which may deepen after even further exposure.
(Left to Right/Top to Bottom)