In order to help the duct tape stick to the fabric of the zipper you need to make the surface a bit rough. I do that by using a sanding block. You can also use sandpaper.
Measure around the circumference of your end piece. This is how long you will have to make the fabric that is attached to the zipper.
Open up your zipper and very carefully place a piece of duct tape up to the edge of the zipper. Be very careful not to cover the tines of the zipper with the tape.
Do this on both sides of the zipper.
Flip the zipper over and do the same thing on the back. Cover the sticky side of the tape with another piece of tape.
Next, continue laying strips of tape on top of each other as if you were making duct tape fabric.
Flip the zipper over and continue doing the same thing. Ultimately you will want to leave one strip of tape uncovered on the back side of your zipper. This will be used when you put the body of the bag together.
Once you have reached your desired length, place a piece of tape half on/half off on the three sides of the fabric which do not have sticky. This will cover any ragged edges and create straight lines.
You are all ready to assemble the body of your bag in Part 3.