Select what you want mirroring and group it and move to one side.
If they are 'axial' use the context-menu to flip - you probably what red or green axis. It can be undo it wrong.
You can also mirror by using the Scale Tool - start stretching in the direction you want to flip and type -1.
Relocate your grouped stuff as needed.
There is also a Mirror Plugin that lets you Mirror about any plane you choose by three points if your orientations are non-axial... Do a Search for it in Plugins...