Mix and boil for 5 minutes (timing is important) the brown sugar, margarine, Karo syrup and salt.
Remove from heat and add baking soda and stir.
Pour over popped corn and mix. Cook in preheated oven for 1 hour. Stir at 15 minute intervals. This process helps bond the caramel to the popcorn.
NOTES: Be careful, this is very hot. Also, I found using one of those disposable baking aluminum pans make clean-up a breeze. Put a cookie sheet under the pan for stability.