you need a router, with a diamond cutter attachment, cut it at the calceous part, don't touch any soft portions or it won't work...
if you don't have the high tech router, or even a band saw...to make it a clean cut...get some type of garden snips...CLEAN THEM WELL...as a last resort...
overall, it needs to be an extremely clean cut across the 'shell' if you will, i repeat..don't touch any soft spots...