Tayammum is done when there is no water to perform the usual Islamic rites, or there is difficulties concerning the water. Tayammum is done using dust when there is no water for ablution (wudhu) or ghusl (ritual wash).

[Quran 5.6] O you who believe! when you rise up to prayer, wash your faces and your hands as far as the elbows, and wipe your heads and your feet to the ankles; and if you are under an obligation to perform a total ablution, then wash (yourselves) and if you are sick or on a journey, or one of you come from the privy, or you have touched the women, and you cannot find water, betake yourselves to pure earth and wipe your faces and your hands therewith, Allah does not desire to put on you any difficulty, but He wishes to purify you and that He may complete His favor on you, so that you may be grateful.

To do tayammum, you first make the intention, then hit the earth with the palms of both hands together, wipe the forhead with them, starting at the hairline and continuing to the top of the nose and eyebrows. Then, the back of the right hand should be wiped with the left hand from the wrist down to the tips of the fingers. The same should be done to the left hand using the right hand to wipe it.