Wash the mantis shrimp carefully.
With a pair of sharp kitchen shears, remove the head.
Cut off the legs and gills at the bottom of the mantis shrimp.
Cut the tip of the telson (or tail) from the back through one of the fins, at a 45° angle.
Trim off the sides, removing the pointed parts (or "thorns") along the shells. Use the edge of the flesh as a guide. Flip the mantis shrimp to the other side, and repeat.
With the mantis shrimp upside on the palm of your hand, hold one side of the telson firmly with your thumb. Gently lift off the bottom of the shell with the other hand.
Then, carefully remove the flesh of the mantis shrimp, and set aside. Repeat the process with the rest of the mantis shrimps.
Once all the flesh have been extracted, they can be cooked immediately, or stored in cling wrap, in an air-tight container, in the fridge for not more than a day.
Add all the shells and any other organic waste of the mantis shrimp into a saucepan of boiling water, and let it boil for 10 minutes.
Once done, the shells can be strained and discarded. The stock can be used immediately, or kept for another day (see Notes).