Remove the old showerhead using adjustable pliers. You may want to wrap the jaws of the pliers with tape to prevent them from marring the chrome. Using a rag (and, if necessary, some paint thinner) to remove any old plumber's putty or Teflon tape left on the threads of the shower stem.
With the old showerhead removed, you can attach the diverter. It's important to make sure that the supplied washer is placed in the neck of the diverter according to the directions that come with the kit (pictured).