Hi Toshi,
Stretching sprue is easy, however not all plastic is alike. I prefer Trumpeter sprue because they typically have thick sprue trees and the plastic stretches well. I avoid using plated or softer plastic.
There are 2 methods that work well for me. One does include setting fire to the end of the sprue so if you try that method be sure to wear a mask. Both methods should be done in a well ventilated area, or next to an exhaust fan like your paint booth.
Method 1. Starting a fire
This is my least favorite but the most effective.
Light the end of a piece of sprue
Blow out the fire and immediately squash the melted end onto something stable.
Pull the sprue to the dimension desired.
Method 2. Heat but no fire.
I prefer this method because there is no burring plastic involved although it is tougher getting the melting point correct. Sorry no pictures for this method.
Holding one end of a sprue, heat the center of a sprue rotating to get even heating. The sprue will soon begin to weaken and go limp. At this point grab the other end and pull slowly. You should be able to continue pulling until the plastic begins to harden. The thickness will be deponent on how much you heat the sprue and the plastic involved. I have been able to get my Trumpeter sprue to a long even width of a hair.
Now here’s the interesting thing, if you need to tighten the rigging, smoke from an incense stick or punk used for fireworks will shrink the stretched sprue. Just be sure not to get to close or you will be redoing the process.
I hope that helps.