Now create a new layer by pressing Ctrl+Shift+N/Cmd+Shift+N. Rename it to “White”. Grab Rectangular Marquee Tool from the tool panel. Hold down Shift and then select the boxes as shown below.
As white is our background color so now we can press Ctrl+Backspace/Cmd+Delete to fill the selected area with white color. Now press Ctrl+D/Cmd+D to deselect the selections.