We used http://pixlr.com/editor to make the negatives for our project. In pixlr, choose "Open Image from computer" and select the photo you want to work on.
Once your image is open, choose "Adjustment" and then choose "Desaturate" to turn it into a black & white photo.
Next, choose "Adjustment" and then choose "Invert" to convert the image to a negative.
You are now ready to print your negative.