Switch back to your photo. Click Edit-->Fill.
For contents, choose to use Pattern. Change the opacity to 50% and change the mode to Darken. Click OK.
Now you're done! Save your chart as a different file name if you want to keep your original photo. Print it out, and now comes the fun part of figuring out which colors to use.... this may take awhile! Stitch Stuff has a great chart of DMC colors and their RGB values. Choose each color in Photoshop to find out the RGB value, and match that up with the DMC colors in the chart.
There are many things you can do differently, some photos may look better when done with a larger or smaller cell size when doing Mosaic. If you change the cell size to a number other than 5, be sure to make your new image (with the lines) the same size as the cell size you used.
If you have any questions or comments, please contact me at email@example.com.
Good luck and Happy Stitching!