Start with a bag that has a design you can cut through and it won’t matter.
Remove the ribbon – I just cut it.
Flatten the bag and cut to the size you want; you need an extra inch at the top to fold again.
Fold the top inch back in; make a crisp fold.
Using a hole punch, make new holes to restring your ribbon. My “new” holes went through the extra folds, but not necessary.
Restring the ribbon, or add fresh ribbon. I’m cheap, I used the old stuff.
Voila, new little gift bag!