Needed materials
Ammonium Iron Citrate (Green)
Potassium Ferricyanide= potassium exacyanoferrate (III)
Hydrochloric Acid (muriatic acid) (optional)

Warning:

Ammonium Iron citrate is available in green and brown variations. We only need the green one.

Do not confuse Ferricyanide with Ferrocyanide. The first one shows as orange-red crystals, while the latter being of a pale yellow color. In the following formulas only the first is needed, which is also used in bleach and reducing baths.

The following formula is considered as a "classic" (Barnier 2000).
Prepare the following solutions:

Solution A
Ammonium Iron Citrate Green	20	g
Water to make	100	ml
Solution B
Potassium Ferricyanide	8	g
Water to make	100	ml

In both cases dissolve the salts in 60 ml of warm water and when dissolved bring the volume to 100 ml with cold water.

Warning:

While the solution B lasts indefinitely, the Solution A has a tendency to form molds on the surface: you can add some salicylic acid as preservative. 0,5 g in 100 ml solution will suffice. Add the acid while preparing the solution A.
Alternatively, prepare smaller amounts of solution to be used within a week.

Both solutions are best kept in a dark cool place.

Paper sensitizing:
Mix the two solutions in a 1:1 proportion.
Be aware to mix only what you need for the paper you are about to treat, considering 4ml for a 20x30cm sheet.
Coat the paper with a brush in ambient light or a small bulb light, avoiding neon light or any source which may contain UV wavelength. Your brush should not be overloaded with solution and the solution should not be allowed to drop or pool on the surface.
Dry the paper in the darkness. The drying process can be accelerated with an hair dryer paying attention not to heat the paper up too much as the solution tends to decompose if heated.

Exposure and development:
The sensitized paper should be exposed as soon as possible.
As the exposure proceeds the paper tends to have a light color shift. When the image appears somewhat solarized, with shadows of a greyish color, it can be interrupted.
Take the paper out the printing frame and immerse it in a tray full of plain water at ambient temperature. Agitate the tray from time to time for about 5 minutes, then transfer the print in another tray again full of plain water and repeat the washing. Wash then your print in running water for about 30 minutes, as if you were processing a normal b/w print.
Adding some drops of hydrochloric acid to the initial washing helps obtaining more brilliant tones and facilitates removal of unused iron salts, which may have the print turn yellow in time.
Once the washing is over, hang the print and let it dry.

Possible defects:

Too soft/too contrasty print: best to make adjustments to the negative, making it closer to the desired result.

The print bleaches during the initial washing, while the water turns blue: in this case the print is underexposed, and the print should be repeated increasing printing time.

The print lightens too much during final washing: water is too alkaline. In this case you should wash your prints by multiple water changes instead of using running water, adding each time a couple of drops of hydrochloric acid to the water.

The print darkens during the initial phases of drying: this is normal as oxygen in the air oxidizes the iron in your print. You can accelerate this process by adding a couple of drops of hydrogen peroxide to the last washing. This step, anyway, has no influence on the final result and so it can be omitted.

Notes:

There are several variants to this formula; particularly, reagents concentration and their ratio determine tonal differences in density and contrast of the print.

Other variations use oxalic acid and/or potassium dichromate as contrast and speed agents.

Cyanotypes are to be printed on several materials including most papers, fabric, ceramic, wood and so on.

Using drawing paper, changing type and manufacturer different results may be achieved.

Barnier J. (2000) "Coming into Focus". Chronicle Books, S. Francisco