How to mirror in Sketchup
Mirror isn`t a standard tool in SketchUp, but there are several ways to Mirror objects using different methods.
There are 3 methods to mirror objects.
Two of them can be done with the standard SketchUp installation, the third one which is preffered method requires a Plugin.
We can select the object we want to mirror, right click on it and select "Flip Along:" This will give us the chance to select which plane (red, green or blue) of the object to use as a mirror plane. The downfall is that you can`t select the mirror plane. The mirror plane has to be one of the planes defined by the center of the object.
We can use the Scale Tool. Select one of the grips (using the grips on the center of the faces of the object's bounding box is the best way to go) and type "-1" as the scale value on the Value Control Box (VCB). Again, we can´t control the mirror plane, it has to be the plane defined by the faces of the bounding box of the object
Option 3 Preferred.
The third method is to use the Mirror.rb Plugin. This plugin is very easy to use and allows us to select the mirror plane and also to choose if we want to keep the original object or delete it after the mirror operation. It is by far the best way to go. To use the plugin (after saving it to your plugins folder) select the object you want to mirror and go to Plugins --> Mirror Selection. Then you will need to pick the 3 points that define the mirror plane, and at the end you will have te option to choose to either keep or erase the source object. Easy and simple, and a time saver.
View the text File Uploaded to visit the Sketchup support link for Mirror Help and to Visit the Site to download the Mirror Plugin.