Open image you want to mirror
Crop as desired – I try to use easy numbers like 4×6 for the math!
Image > Canvas Size > put in the height or width that you are doubling depending on whether a vertical or horizontal shot – for instance, if you have a horizontal 4×6, then put 6 in the width and click on the left or right pointer in the center row of the position tool. I use white for the extension color but it does not really matter as you will fill that in.
Photoshop will add extra space below the image in this case.
Click in the white space to select.
Ctrl/Cmd Shift I to inverse and select the bridge photo.
Ctrl/Cmd J to put a copy of the bridge on another layer.
Drag the new image to the white area.
Edit > Transform > Flip Vertically.
You can also flip both layers vertically again to get a different look.
On the layer with the white, use Magic Wand W to select the white and delete those pixels with Ctrl/Cmd X.
If you have trouble with the background layer, double click the lock to make it editable.