This recipe from Rick Stein’s Far Eastern Odyssey is useful instruction if you’ve never made tamarind water before. After you learn the technique it’s easy to make a small quantity as needed for recipes unless you find yourself going through lots of it.
Take 60 g tamarind pulp (about the size of a tangerine) and put it in a bowl with 150ml hand-hot water. Work the paste with your fingers until it has broken down and the seeds have been released. Strain the slightly syrupy mixture through a fine sieve into another bowl and discard the fibrous material left behind. The water is ready to use and will keep in the fridge for 24 hours.