Here are a couple of images that show how to fit external mouthguards to snorkels. There are lots of different snorkel designs so think carefully before you cut! And remember to cut away less than you think you need - you can always take more off but it's hard to glue anything back on!
NB - the flatter part of the mouthgard should be at the top - the pink snorkel is actually upside down in the picture.
Sometimes a cable-tie can be useful to secure the mouthguard in place but make sure there are no sharp edges left after trimming the excess off. For this snorkel (below) two small holes were drilled in the mouthguard to thread the cable tie through.