Step 4: Okay, now the fun stuff starts
To do a photorealistic mosaic, you would then just print it out, cut it out if necessary, and tape it into the back of your frame, like you were framing your template. I warn you now that I don't do photorealistic style because it is HARD. Really hard. you need tiny pieces, lots of colors, and more paitience than you would think or have. (Unless you do, in which case, damn you are dedicated.)
To do a stylized mosaic like mine, use a filter or hand draw over the photo using tracing paper. I use the "cutout" filter in photoshop and usually go with 2-4 layers depending on how many colors I want to use. Play around with it to see what you want to do, and it helps if you pick your theme of colors now. With mine I went with black/white/red and a little green.
Once you have your template finalized, print it and tape it into the back of your frame securely.