If you have a portrait taken on a plain background and you want to extend the background there are a couple of ways you can do it in Photoshop but one of the easiest ways is with the Crop Tool. You may also find it useful to have a rule of thirds grid to help with the composition of your image.
Step 1: Crop Tool
Select the Crop tool and draw a box around your image, extending out to where you want the canvas to finish then hit OK on your keyboard. Your image should now have space to which ever side you extended it on.
Step 2: Fill the layer
The canvas is now extended but as it's not filled in it looks a little odd. To fill it in, select the Rectangular Marquee tool and draw around part of the background close to the new area that needs filling in, taking care not to include your subject within the selection. Then go to Edit>Free Transform and several anchor points will appear around your selection. You can drag these left,right, up and down to fill the space then hit OK once you've completed the transformation.
Be careful how much you stretch the background though as it can appear distorted and won't blend with the original shot very well. If you find this to be a problem, we found the Smudge tool was useful for blending away any marks. You could also only extend the canvas to a point where it doesn't distort/stretch, go to Edit>Copy, Edit>Paste then using the Move tool, slide the shape into position next to the first piece of background you placed, repeating the process until the background's complete.
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