This is just as important as the developing, stop bath and fixer. The temperature of the water needs to be close to that of the previous process of 20 degree C.
The purpose of washing is to remove the last of the fixer from the emulsion and needs to be done thoroughly to ensure the longevity of the negatives.
Which method to use? One way is to empty the tank and refill with fresh water and agitate for a few seconds, this should be repeated several times. Another way is to use a wash hose that is specially made to fit into the top of the developing tank and attached to the tap. I personally favor this approach with my own refinements: after fitting the hose in place I open the tap till it is nearly fully open and leave for thirty seconds, this vigorusly removes the last of the fix that remains. Then slow the flow right down for the next ten to fifteen minutes. It is a simple straight forward procedure that has served me well over the years.