Mandatory Disclaimer: Do not be stupid.
The following is a video showing high-level instructions on how to build an almost exact copy of The Boring Company's flamethrower. Basically, install this $ 125 Airsoft gun and insert a propane torch. The total bill for the goods should be less than $ 200, well under $ 500 that the boring company billed until it was sold from the 20,000 units.
As many have pointed out before, The Boring Company launches a flamethrower that confuses the definition of a flamethrower. It is rather a large torch like those used for roofing or to clean undergrowth or melt ice.
It does not matter. It's good to have fun sometimes.
Elon Musk said after launching a Tesla Roadster in the space, "It's a bit silly and fun, but I think stupid and fun things are important." I agree. This DIY flamethrower is stupid and fun and it is the kind of project that could inspire and engage someone in the discovery of mechanical engineering. Do not be silly. Please.