Measure down 2 inches from the top edge of the bottle and make a mark with your Sharpie. Turn the bottle and make a few other marks at the two inch line.
In order to make a clean cut we are going to create a line with duct tape. Cut a piece of tape the length of your bottle. Cut it in half lengthwise. Place this piece of tape at the two inch marks. This becomes your cut line.
Use your box cutter to cut along the top edge of the duct tape. Cut the top off of the bottle. Save that piece. We will use it for a later project.
If your cut is uneven use your scissors to trim. Do the same with a second bottle. You will need two pieces that are exactly the same size.
Lay out your zipper. Cut a piece of duct tape the length of your zipper. Cut that piece of tape in half length wise.
Starting at the closed end of the zipper carefully add some glue and glue the zipper to the plastic bottle. You will want to glue just a few inches at a time. Be careful to not get glue into the teeth of the zipper.
Once you have glued the zipper onto both sides of the bottle use the two pieces of tape you cut earlier to secure the zipper onto the bottle. Again, be certain to not tape over the zipper.
Open up your bottle and add a few small pieces of tape where the bottles hinge together.
Your pencil holder is complete. You can embellish with additional tape, stickers, or whatever you have on hand.