Here's a thread that may help you.
Forums at Splitcoaststampers
I purchase candy bags from my local AC Moore. They come in various sizes. I measure the width of the bag, add 1/2", then determine how high I want the topper (usually 2-3 inches), double that measurement, and cut my paper.
For example: If my bag is 4 inches wide, I'd cut the paper 4 1/2" inches wide. If I want the topper to be 3 inches tall, I'd cut the height to be 6 inches, thus making the topper 4 1/2 x 6. Decorate, fold in half and attach. I usually staple mine on. Simple.
Hope this helps.