like the face ,they should also always be washed from top to the bottom and if washed from below to above the WUDU will be void.
Pour water over the right fore-arm from the elbow to the finger-tips; and using the left hand, wipe the water over the arm to ensure that all the necessary parts are washed. Then do the same with the left fore-arm. The washing must be done from the elbows to the finger-tips and not vice versa.
The water should be poured from a little above the elbow to ensure that the whole fore-arm is covered. It is necessary to wash the fore-arms in such a way that the water penetrates the hair, if any, and reaches the skin.
The right fore-arm should be washed before the left.