To be honest, it's easier to show than to describe or explain. If you google stretching sprue
you should find a video somewhere.
But, briefly, you need a short (5cm should do) length of straight sprue and a heat source.
A candle (adequately secured to prevent accidents) is fine. Light the candle. Hold the sprue
at each end and position it just above the candle flame, watch the sprue carefully, keep a slight
tension on the sprue and you will notice it soften and the surface become "glossy". At this point,
remove the sprue from the heat source and simultaneously pull the ends of the sprue apart.
You should find that the soft glossy sprue will stretch into a thin filament like a length of wire.
It takes a little practice, but once you get "the knack" of it, you can control the thickness of your
product by varying the speed and extent of your "stretch". I think the problem most beginners
have is pulling the sprue without removing it from the heat. It will just break and maybe catch
fire as well! It's a useful technique to learn to create antenna wires, rigging, brake lines etc.
Good luck with it!