Don't tell me you have lost it!!!
If anyone is interested. This is the tut.
New Image - 300 x 300. Transparent.
Activate mirror - copy and paste into the new image.
Click on Layers - New raster layer.
Activate the subject.
Copy and paste - so it is to the left of the mirror.
Click on Layers - New raster layer.
Activate the original subject.
Click on Image - mirror.
Copy and paste into the mirror.
And that's all there is to it, folks.