Find an appropriately sized cylinder (I use an AA battery) and wrap flash paper around it, gluing the tube closed with nail polish. After the polish is dry, remove the cylinder.
Insert the igniter assembly so that one string handle comes out each end of the tube. Insert the toy in the tube. Optionally, stuff in some non-flammable confetti — but not so much that it will interfere with the ignition of the tube. For a better grip, you can tie a small ring to each handle.
Twist the ends of the tube closed. Make sure the handle strings are not mechanically attached to the tube. You don’t want the tube to pull apart like a normal cracker; it should remain intact and be ignited along with everything else.
Stand away from anything flammable and keep the cracker away from people and pets. With one person holding each string, stand apart, and on a count of three, yank the strings sharply to pop it. Don’t be timid: if one person gives, it might just pull the handle strings off.