Strategically, in terms of combat, what's the point of a flaming sword? Seem like mostly it would increase your chances of hurting yourself. I guess a hot blade might be able to cut through something better, but then wouldn't you rather have a heated sword than a sword with some flames coming out of it? Even the flaming sword in Fallout 4, Shishkebab, doesn't do that much damage.
But it looks really cool. I think that's what flaming swords are best for: scaring people and feeling badass. Just look at Make's Caleb Kraft, who made his own, real life Shishkebab. Are you not intimidated? A little jealous maybe?
If you are, in Make's video, Kraft explain how he made the sword using an ignitor from a propane barbecue grill, a can of butane, and a brake handle from a motorcycle… though you probably shouldn't try this. Please don't try to make a flaming sword. Be safe and admire Kraft's work instead, or just play more Fallout 4. You'll like it if you like Fallout. This video will show you where to find Shishkebab in the game.