Here's how I'd do it.
Step one: Put an aspect on your character sheet; something like "I've got a lightsaber!" - this lets you spend a fate point any time you need to do something really cool with it, and, conversely, gives you fate points when what you really needed was a crowbar and a real sword would've been a better choice - or when you need to convince someone that you come in peace, unarmed, and the guys you're talking to know better.
Step two: Buy an appropriate sponsored magic - maybe you've cut a deal with Pele (Hawaiian goddess of fire); maybe you just found a nice fire-aspected ley line nearby and spent the time and effort to learn how to tap into that and cart its power around with you. Lots of options available here. As an added bonus, this provides an instant explanation for some PC going "Why can't I do that?", as well as a good source of possible quest material if said PC decides they really want to be able to replicate this trick.
Step three: Conjure the sword. This can be done as an evocation, thanks to sponsored magic.
For a weapon: 2 blade (i.e. your average sword), it's a complexity 3 thaumaturgy spell, which translates to a power 3 evocation, and it'll naturally last until sunrise. Of course, you want something more impressive than that, and you don't need quite so much duration; I'd probably give you weapon: 3 for a scene as a complexity 5 conjuration - that's on par with Item of Power type swords, which is about where it ought to be.
Then, since it's thaumaturgy, you start adding maneuvers; three more complexity for every aspect you want to put on the blade. Put on as many of these as you can afford.
In combat, you start off by just using it as a sword and taking your free tags on those aspects when you need to make it do something special; if the free tags run out, you can cast again to "reinforce" the blade (since it's already conjured, this'd just be however many maneuvers you want at 3 power per each, which you can do for sure thanks, again, to sponsored magic).
Or you can start spending fate points to invoke your "I've got a lightsaber!" aspect for effect. I'd require that if you want to do something really dramatic, like melt the steel weapon someone tried to parry with (remember, the star wars universe is full of non-lightsaber melee weapons that seem to be capable of parrying a lightsaber just fine. And while parrying will certainly *damage* a metal blade, it takes quite a lot of heat indeed to render one instantly broken.)
You don't actually need a handle, though were I statting up such a character, I'd definitely make a "lightsaber handle" as a focus item for fire evocations.
