Heat one cup of water to a boil and pour it over the tamarind. Stir, then soak the tamarind for 10 minutes.
After soaking, mash the tamarind well until it feels completely soft. Remove any hard fibres, seeds or pod fragments.
Strain the tamarind water into a fresh bowl and squeeze out as much extract from the pulp as possible. Set tamarind water aside.
Return the pulp to the first bowl. Heat ½ cup of water to a boil, pour over the pulp and allow to steep for another 10 minutes.
Strain this second extract into the earlier one. Again, squeeze as much liquid from the pulp as possible.
Discard the pulp. The tamarind water is now ready for use.
notes
Tamarind paste is the fleshy pulp of the tamarind pod. Ensure there are no seeds or heavy fibres. A golf ball sized amount of this pulp weighs around 50g. You can buy tamarind paste from a good Indian grocer. The post on tamarind discusses this in more detail.