I recommend setting your net in clear conditions in the evening or during the day in muddy water after observing where the mullet are moving.  Mullet actively leap and writhe through the water before crossing estuaries and mudflats on the rising tides in schools.  Most commonly mullet are found in shallow water as they feed on microscopic plant life and small creatures that reside in the mud or sand.  Their long intestine enables them to eat a mostly plant-based diet.  Stealth is important catching this wily fish, as they are easily scared away.  Set your net in knee-deep water as the tide comes in, dusk is a good time especially since mullet have good eyesight and may otherwise spot your net.

Remember that if you are looking for larger fish having a net with the mesh at the smallest legal size allowed can be counterproductive.