Leave the milk (usually from cattle, yaks, camels) to curdle. Lift out the solid components with a fine cloth and let as much of the liquid drip off. Then press the mass into a cake of a few cm height between two wooden boards, weighted down with stones.
Cut the solid cake into pieces of about 10 length. Arrange the pieces on a wooden board and put them into the sun for drying. In Mongolia, this happens on the roof of the yurt. A cover of fine white cloth will keep the birds away.
The dried pieces can be stored almost indefinitively. They can get quite hard, so most people rather suck than bite on them. The taste may vary regionally and depending on the milk used, but usually includes a combination of sweet and sour.
Arul belongs to the most common travel provisions (next to Borts). The pieces are also a ready snack for the small (or larger) hunger at almost any time. Some sources cite Aaruul as the primary reason that traditionally living Mongolian people have very little troubles with their teeth. It is also one of the core vitamin sources for the nomads.
“Worm Aaruul” is a variation in the shape of little strands. Don’t press the fresh material, but put it through a meat grinder (available in every Mongolian houshold) into small “worms”. Arrange those in little heaps for drying. This type of Aaruul is easier to chew (especially for children), but less suited for travel supplies.