like in this way ​

(think that the big "G" is any blocks and "W" is water and you need to get the water from the "N")​

(You don't need to put the water in the "N" because there will be filled by the water naturaly.)​

G​

G W G​

G N G​

G W G​

G​

(but just get water from "N") Get the water with the bucket ​

​

 

You can also create an infinite water source by doing the following:

B B B B
B W N B
B N W B
B B B B

W = Water
N = Nothing
B = Blockd

 

This was common knowledge since alpha. This is actually pretty basic