From SpecialAttack World Of Minecraft

Summary

/mirror <axis>

Alias: /flip

Mirrors the copied blocks along specified axis. More than one axis can be given. Axes are X and Y.

etc

Using Mirror

Finding the three key marks

Copy point 1 will be at the front-most bottom corner away from the mirror plane.

Copy point 2 will be at the back-most top corner against the mirror plane.

Paste point 3 will be in the bottom-front and against the mirror plane.

Using the compass

Face your build so that the mirror will be going either right or left.

Type /compass.

If you are facing either North or South, use /mirror x. If you are facing either East or West, type /mirror y.

Other examples

Mark #1 at xx	Mark #1 at xx	Mark #1 at xx
Mark #2 at xx	Mark #2 at xx	Mark #2 at xx
Paste mark at xx	Paste mark at xx	Paste mark at xx
screen	screen	screen

etc