Ive done it a few times, mostly on my Garand.
You want to have a strip of the same material your welding to act as a filler rod. Then heat both sides of the joint and fill with the filler strip, then weave your iron back and forth creating welded beads. Theres alot of filling, cooling, sanding, filling going on. It takes a little time but once you get the basics down you progress faster. Dont forget to wear a mask, melting palstic causes bad fumes.
Also make sure you fill the joints to helps trengthen them. My friend had a busted m14 stock (broke at the hand grip area, exposing the motor) I plastic welded the stock back together and it hasnt had a problem since.