Preheat oven to 250°F. Have a 15×10-inch roasting/jelly-roll pan ready (you can leave it bare or line it with non-stick aluminum foil).
In a large, heavy-bottomed saucepan combine 2 cups light brown sugar, 1 cup margarine or butter, and ½ cup light corn syrup.
Heat the mixture over medium, stirring until the margarine/butter melts and the sugar dissolves. Increase heat slightly and bring to a full rolling boil.
Once boiling, set a timer and boil for 5 minutes, stirring often to prevent scorching.
Remove the pan from the heat and immediately stir in ½ teaspoon baking soda. The mixture will foam and lighten in color; stir until the soda is fully incorporated.
Pour the entire 20.5 oz package of crunchy Cheetos (original) into a large heatproof bowl. Pour the hot caramel over the Cheetos and gently stir with a spatula or wooden spoon until the pieces are evenly coated. Work carefully—the caramel will be very hot.
Transfer the coated Cheetos to the prepared 15×10-inch pan and spread into an even layer, breaking up any large clumps with the spatula.
Bake at 250°F for 1 hour, stirring and redistributing the Cheetos every 15 minutes so they brown and set evenly.
Remove the pan from the oven and immediately spread the Cheetos in a single layer onto non-stick aluminum foil to cool. Once they have started to set and are cool enough to handle, break into pieces. Let cool completely before storing.