Summarize:

The marshmallow goes into the 7 in (18 cm) end, while you blow into the 3 in (7.6 cm) end. Make sure that you take a deep breath before you blow into it!  Never aim at animals or people’s faces. It’s not recommended that you aim at other people. If you do decide to have a marshmallow fight, wear some safety goggles to protect your eyes, just in case.
Pop a mini marshmallow into the gun and blow on it.