A Shield Bug, once released, has no wish to go back into their transformation place, and it's only desire is to protect its releaser from whatever enemy they may have acquainted.
Shield-shaped charms, which can be found in the book Shield Bug Preserves: 500 Charms.
Following the recipe in the above book, stir your mixture into preserve pots. (You will know that the mixture is ready when it smells something like boiled rat and rotten fish.)
Pass the shield charms around the group, so that each member touches it, then put the charm in your first preserve pot. Pop a bug in the same preserve pot, making sure to get the lid on quickly and screw it tight.
Repeat until all your preserve pots contain a bug. Watch and wait as these normal insects become shield bugs! When each bug has stout, armored green wings, and their front are covered in heavy green scales, tap on the jar with the silver hammer found in your spell book.
The bug is then ready for release, whenever you may need it.