I have done a bit of this.
Its stinky, toxic, and easy to make mistakes.
Best done with electric or hot air heat sources, not flame.
They make hot air welding guns for plastic, which while not cheap, are probably worth it, as there isnt really an easy substitute.
You can cut strips of filler rod from your same plastic.
You need to be able to finely focus very hot air.
Theoretically, you might be able to make a tip for a heat gun, but a decent heat gun costs a hundred bucks or more, and by the time you make a tip, you are getting close to a real welder.
The other thing I have done a lot is using resistance heaters to heat a line in plastic, then bend it hot.
These heaters are cheap, and 110volt.
Plastics are not very esthetically pleasing to work with- they are, after all, pretty much pure petroleum.
Which is one reason I liked working with them a bunch in the 70's- they were so anti-hippie, they offended nicely.
But ultimately, not satisfying.
Use a respirator, and good ventilation- they do nasty things to your innards.
Being oil based, the prices have gone up a bunch lately for new. Course, milk jugs are a good source of free material.