Welcome to Star Wars month, everyone, it's going to be a long road filled with spoilers, polarized opinions (opinions are ALSO spoilers!), lightsaber-fied Facebook profile pictures, and probably a hundred different homemade takes on the series' iconic weapon, many of which you'll probably be sick of by the end of this. Allow me to add one that might just be a little more badass than dorky. It's a mini flamethrower.
This video from Allen Pan of YouTube channel Sufficiently Advanced shows his flame-propelling "lightsaber" in action as well as a breakdown of the parts he used to cobble it all together.
In short, it appears he made a metal enclosure that acts as a small tank for the methanol-acetone fuel mixture. There's a syringe valve toggled by a button on the saber, and that button also seems to ignite the fuel and play that iconic lightsaber activation sound effect.
Unfortunately, because it's a stream of fire, you can't really expect it to keep its shape all that well when swinging it. But Pan still makes good use of it by popping a bunch of Storm Trooper balloons and setting Jar Jar Binks on fire.