Open the resize-practice-small.jpg image you saved from our last exercise and choose the Canvas Size command. (Image > Resize > Canvas Size.)
Let's practice by adding a half inch to all four sides of our resizing practice image. The Canvas Size dialog shows the current size as 8.333 inches wide and 6.25 inches high. With the "Relative" box unchecked, you will need to increase both the height and width by one to add a half inch to each side. Go ahead and enter 9.333 for the width and 7.25 for the height, leave the middle anchor square selected, set the canvas extension color to white, and click OK.
You should have a new white border all around the image. Now choose Edit > Undo (or Ctrl-Z/Command-Z) to undo the change and we'll try it using the relative option and a different extension color.