Decorative Edge Scissors
Japanese Masking Tape
Ornament ( optional)
Cut out two strips of masking tape the length of the opening of the bag. Trim the edges with decorative scissors. Place the tape on the bottom of the bag so half of it's hanging off. Fold the tape that's hanging off over to the other side. Fill bag with contents. Tape ornament to the top. Place the other strip of masking tape on top of the bag just like you did for the bottom. Done.
Here are a few more ideas to get you inspired.