Let's start with this picture.
Next step is working on the extension of the background. Click on C for the crop tool. Enlarge the background and make sure the 'Delete cropped pixels' is off.
Next step is selecting the transparent part of the image and filling it, using the content aware fill. To select use the Magic want tool (W). To fill it go to Edit > fill and choose Content aware
So there you have it: more background and the image went from portrait to landscape. I guess overall it's sort of OK, but there's still some work to be done at the bottom of the image.
You can work with the Patch tool to erase the unwanted (J). Or you can use another technique to fill the transparent part of the image: the Rectangular Marquee tool (M) to make a rectangle of part of the background
Next is expanding the rectangle using Cmd T and dragging the right slider to the right.
Voila, this looks much better right?
To even it out, the image now needs a bit more space on the left side too, but I guess you know how to do that now.
To give you even more info: here's a short Youtube video on the subject:
picture from pixabay.com