The components of this kind of ink might bother your skin, be careful, and protect yourself.
The following recipe is based in a formula you can find in one of the best books you can have about calligraphy, "Caligrafía. Del signo caligráfico a la pintura abstracta" by Claude Mediavilla. 
You can buy the book directly from the Campgràfic website.
Ingredients
Tannic acid — 5 g
Iron sulfate — 3 g
Gum arabic powder — 5 g
Water — 1 liter
Instructions
All of the ingredients must be finely powdered, if you got them in pieces you must pulverize them separated in a mortar.
Mix the iron sulfate with 600 ml of warm water.
Mix the tannic acid with 200 ml of water and boil it for half of a minute.
Let the water with iron sulfate and the water with tannic acid get cold and then mix them (don’t worry, it will get darker at this point because of oxidation).
Mix the gum arabic with 200 ml of warm water and add it to the mix (if you like more viscosity you can add a little more gum arabic).
If you want a darker ink you could add extra fine carbon black.
That’s all, now pack your ink in your preferred container and use it for your practice.
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