You have just assembled a miniature hydroponic system. Cover the can and duct tape well. Shake the can to mix. Put a layer of ice and rock salt in large can. Add small can and layer more ice and rock salt.
Cover can and tape well.
Kick or roll the can for about 10 minutes. Remove the small can. Scrape ice cream away from the side of the can. Put back in large can with more ice and salt. Kick or roll for 5 more minutes.