Flatten the bag
Fold the bag into thirds along the length of the bag (decreasing the width)
Fold the bottom of the bag up one third
Take the handles of the previously folded bag and loop them around the active bag (Skip this step for the first bag)
Fold the bag up another third
Stuff into empty tissue box with the handles of the bag poking out of the top
Repeat steps 1-6 until the tissue box if full or you run out of bags