Correct mixing of Fine Casting plaster for mould making and casting. 

Ratios are:

1 Litre cold water - 1.3kg dry plaster powder
2 Litres cold water - 2.6kg dry plaster powder
3 Litres cold water - 3.9kg dry plaster powder
etc...

Plaster can be bought from CCF in Canning town, Travis Perkins, Tirantis or many other Art Shops