Click the color photo in the layer palette, add the channel mixer adjustment layer, check the "Monochrome" checkbox in the Channel Mixer dialog box, adjust the sliders for the best results, and click OK.
Note: If you only have Photoshop Elements, you can use a Hue/Saturation or Gradient Map adjustment layer to convert to grayscale. Both of these methods are described in my tutorial on Selective Colorization.
