how to use symetry in my rectangle geometry
 

i am new in gambit

i have a rectangle geometry with velocity inlet at the inlet(face 1) and outflow at the outlet(face 2) ,the other faces(faces 3,4,5,6) are wall. i want to use only 1/4 of my geometry by using symetry. how to do this? i use axis or surfaces? and which surface or axis? give me the procedure step by step please best regards