As soon as you have removed your heated plastic rod or straw from your heat source, gently begin to pull evenly on either side of the area of softened plastic. Keep Your Fingers Away From the Heated Plastic to Avoid Burns!
As you gently draw the ends of your rod or straw away from the heated area, the straw or rod will narrow. If you are making a series of bottles which need to be roughly the same shape and size, you will need to draw or stretch your bottles out against a ruler so each section is pulled apart by roughly the same amount. You will also need to start with a standard length of straw or rod. Experiment with your technique, you may need to gently turn the plastic straw or rod as you pull it out, to prevent the neck you are forming from slumping to one side of the rod or straw. As much as possible, you want to keep the narrowed column of plastic centered on the main width of the rod.
When you have drawn the plastic out to a neck length and thickness which suit the type of bottle or jar you are making, carefully hold the plastic under the same gentle tension until the neck area cools and sets in place. When the plastic is cool enough to set, leave it aside to cool, supporting it evenly on both ends.