Unplug the lamp and examine the plug. If both prongs of the plug are the same size, you'll be able to cut the wire on either side of the cord to install the switch. If one prong is larger than the other, however, this indicates a polarized plug. To add a switch on a cord with a polarized plug, you'll need to cut the wire on the side of the cord opposite the larger prong. In this demonstration, the white plug is standard, the brown is polarized (Image 1).
Once you've determined which side of the cord you'll need to cut -- or if either side can be cut -- you're ready to use your utility knife. Begin by making about a 5/8" slit in the center of the cord, between the two wires (Image 2), taking care not to damage the insulation.
Next, pull the wires of the cord apart to create a diamond/marquis-like shape (Image 3).