Heat up your soldering iron, outside. Put on the soldering respirator, per manufacturer's instructions.
Touch the end of your soldering iron to the area of your plastic that you want to melt. If you are welding two pieces of plastic, place them adjacent to one another, clamp them together, and touch the soldering iron to the groove between the two pieces of plastic, allowing them to melt.
Move the soldering iron over the two pieces of plastic. Add plastic filler to the plastic as you move the iron, to avoid warping.