Tayammum is also an act of worship consisting of wiping the forehead and the two hands. It is a substitute for wudu' and ghusl. The Qur'an says: O you who believe! ... If you are sick, or on a journey, or one of you has come from toilet, or you have `touched' (i.e., had intercourse with) your women and you cannot find water, then you should do Tayammum on the pure earth by wiping a part of your face and your hands. (4:43, also see 5:6)
B. MANNER OF PERFORMING TAYAMMUM
After the niyyat for Tayammum, do the followings:
l. Strike both palms onto the earth.
2. Wipe the palms together over the forehead from the hair-line up to the brow and above the nose. "Above the nose" means up to the bridge of the nose. Eyes, nose and cheeks are. not to be included. Should the complete palms of both hands wipe the forehead? No, it is not necessary
that the entire palms of both hands should wipe the forehead; the important thing is to make sure that the entire forehead has been wiped.
3. Then wipe the palm of the left hand over the back of the right from the wrist down to the fingertips. Then do the same with the right palm on the left.
4. Then again strike both palms onto the earth.
5. Then repeat the acts mentioned in No. 3 above.
This manner of Tayammum is based on the ahadith which have elaborated on the words of the Qur'an which say, "do Tayammum...by wiping a part of your face and your hands."
C. THE THINGS ON WHICH TAYAMMUM CAN BE DONE
The verse of Tayammum mentioned above, says clearly that "do Tayammum on the pure earth." Based on this verse and the explanatory ahadith, our mujtahids say that Tayammum should be done on one of the following forms of earth (in order of preference):
1. Earth (fine or compacted).
3. Pebbles or stone-other than mineral or precious stones.
If no form of earth is available then, and only then, one can perform Tayammum on the dust (that had gathered on the floor or the ground, on a carpet or a cloth). If dust is not available, then mud can be used but in such a way that after the hands have been placed on it, they should be cleaned by rubbing them together.
All the items of Tayammum must have the following conditions:-
1. It must be dry as much as possible.
2. It must be tahir (pure).
3. It must be mubah (lawful).
4. The place where the above mentioned things are also must be mubah.
D. WHEN TO DO TAYAMMUM?
Tayammum can be done in the following seven circumstances:
1. When enough water cannot be obtained for wudu' or ghusl.
If there still is ample time for performing salat, then one should wait and pray when he reaches a place where water is available.
When water cannot be obtained, is it obligatory to search for it?
If one knows that water is not available, then it is not obligatory for him to search for it. But if there is a probability of finding water, then it is wajib to search for it until one becomes sure of its unavailability. In the latter case, if one is in plain and clear land, then he should search for a distant of 400 steps in two directions; if he is in a hilly area or in a forest, then he should search for 200 steps in all four directions. However, if one is sure of water's unavailability in a certain direction, then it is not necessary to search in that direction.
2. When water is available but difficult to reach.
It does not make any difference whether this difficulty is physical or otherwise. Therefore, if reaching for water involves danger of life, reputation or property, then one should do Tayammum. For example: owing to old-age or illness it is difficult to reach to the water, or in going for water a person is endangered by animals or thieves; or the owner demands outrageous price for water, etc.
3. When use of water for wudu' or ghusl is dangerous to one's health or life.
For example: one who fears that using water might make him sick or prolong his illness, then he should do Tayammum. However, if use of warm water in such a case is harmless, then Tayammum cannot be substitute of the wudu' or ghusl.
4. When water is available but one is afraid that by using that water he will put himself, his companions or his domesticated animals in danger of thirst. In such a case, one should perform Tayammum instead of wudu' or ghusl.
5. When the time of salat is so short that if one starts to do wuda' or ghusl his prayers will be qada whereas by performing Tayammum he will be able to say his prayers in time. In such a case one should do Tayammum.
6. When the body or the only available cloth of a person is najis; and he finds that if he uses the water for wudu' or ghusl, his body or cloth will remain najis. In such a case he should first purify his body or his cloth with the available water and then do Tayammum.
7. When the use of water depends on those things which have been forbidden by the shari`ah. For example the water has been obtained without the permission of the owner, or it is in an unlawful (ghasbi) utensil or the utensil is made of gold or silver in which one cannot do wudu' or ghusl. In all such cases, one should do Tayammum.
E. THE CONDITIONS FOR THE VALIDITY OF TAYAMMUM
Similar to what you read in wudu and ghusl, the validity of Tayammum depends on certain conditions. These conditions are five in number:
1. Niyyat. If the Tayammum is only one, then it is not necessary to specify it whether it is a substitute of wudu or ghusl.
2. Continuity (muwalat). The acts of Tayammum must follow each other.
3. Order (tartibi). All the acts must be done in the order described above.
4. The parts of the body relevant to Tayammum (i.e., fore-head and the two hands) must be tahir and there shouldn't be any type of cover on them, e.g., ring, nail polish, etc.
5. Under normal circumstance, a person must do Tayammum by himself. But in case of disability, someone else may help him. In the latter case, the helper should take the hands of the helped and strike them on the earth and do the Tayammum; if this is not possible, then the helper should strike his own hands on the earth and then wipe the fore-head and the hands of the helped.
F. SOME GENERAL RULES
If there still is ample time for prayers, then one is not allowed to perform his salat with Tayammum unless he becomes sure of water's unavailability.
What happens if water becomes available while one is performing his salat with Tayammum?
If water becomes available while one is performing his salat with Tayammum, then there can be two different circumstances: (1) The water was found after he had already gone to the first ruku`-his salat is valid and there is no need to repeat it. (2) The water was found before he had gone to the first ruku `-he will have to repeat his salat with wudu. This rule is based on a question which Zurarah had asked Imam Muhammad al-Baqir (as): "What should a person do if water comes while he has already started his salat (with Tayammum)?" The Imam said, "As long as the person has not yet gone to ruku`, he should leave his prayer and do wudu ; but if he has gone to ruku `, then he should continue his prayer. Indeed, the Tayammum is one of the two purifications." 
What happens if water becomes available after one has performed the salt with Tayammum?
If water becomes available after the salat has been performed, then it is not wajib to repeat that salat with wudu'.
Tayammum is a sufficient purification; a person who has done Tayammum is permitted to do all those things whose validity depend on wudu' or ghusl, e.g., entering a mosque, touching the writings of the Qur'an, etc. This is valid for as long as water is unavailable; once the water become available, Tayammum automatically becomes invalid.
If more than one ghusls are wajib on a person, then a single Tayammum with the niyyat of all those ghusls will suffice.
A person on whom ghusl janabat is wajib has to do one Tayammum instead of the ghusl; there is no need for him or her to do another Tayammum for wudu'. But if a ghusl other than ghusl janabat is wajib on that person, then he or she has to do two Tayammums: one instead of the ghusl and the other instead of wudu'.
. Wasa'il, vol. 1, p. 991-2.