Found this on the web.
It was posted by "rosieclown" on another home site.

Caramel Popcorn Kit in a Jar

1 can Sweetened Condensed Milk
1 1/3 cups un-popped Popcorn
1 cup Sugar
1 cup packed Brown Sugar

Set aside can of milk. Layer sugar and brown sugar in a one quart canning jar. Next, place popcorn into a small zip baggie. Seal baggie and place on top of sugar. Place lid on jar, then using clear packaging tape, attach the can of milk to the bottom of jar so they are firmly connected for storage purposes & store in a cool dry place until ready to use. Attach the following instructions for later use, or for gift giving:

Caramel Popcorn
Remove popcorn from jar and using your preferred method, pop corn until you have about 12 cups or 3 quarts of popped corn. Remove un-popped kernels from corn, and set aside. In a large saucepan, mix sugar from jar with 1/2 cup butter or margarine and the can of sweetened condensed milk. Stirring constantly, bring mixture to a boil. Boil for one minute and remove from heat. Working quickly, pour mixture over popped corn, coating as much as possible while pouring, then using a large wooden spoon, mix popped corn and caramel until all corn is well coated. Spread onto cookie sheets which have been prepared with non-stick cooking spray, or eat directly from bowl.