Hello. This is Ochiai, the designer.
I'd like to introduce a handy Photoshop feature for fixing photos that came out slightly tilted.

The feature we'll use is the "Ruler" tool.

It may be hard to spot, but the "Ruler" tool is in the same place as the Eyedropper. First, select it.

Next, use your mouse to click and drag a line from one edge of the photo to the other.

At this point, draw the line along the slant, in a spot where the tilt is easy to see.

Once you've drawn the line, select "Image," "Image Rotation," and then "Arbitrary" in that order.

The angle is then displayed automatically, so just click "OK" as is.

The angle of the photo has now been corrected. Photoshop automatically calculates the angle needed to make the line you drew horizontal, so you can level the photo far faster than rotating it yourself.

All that's left is to fill in the edges that came up short due to the angle correction, and the fix is complete.

Next time, I'd like to show you an easy way to fill in those missing areas.