The person with the gloves should use their hands to compress and mold the contents for at least a minute. If you have more gloves, get others to help. If the person holding the bag shuts it too tightly, the bag starts to blow up, or inflate. This is because some of the carbon dioxide is sublimating , or turning from dry ice into carbon dioxide gas. If a comet's orbit takes it near the sun and the sun heats it up, the surface of the comet starts to sublimate and break down.
Some comets go so near the sun that they completely evaporate and burn up.
Don't worry if your comet doesn't look round and smooth. Real comets aren't either. To see at a real comet passing by the earth, go to the Gallery and look at the close-up picture of Comet Halley taken by the Giotto spacecraft in 1986. This steam is really water vapor that is condensed by the super-cold CO2 sublimating from the surface. Comets spend most of their time as frozen globs traveling far away from the sun on huge orbits, that may or may not go near the sun.