Remember when you were building your gaming rig and you had to travel to the village atop a Himalayan mountain to receive a graphics card from a priest of Cthulhu who was thanking you for bringing her son half way round the world to be presented as a sacrifice to the ancient ones? Well, to craft a legendary item in Guild Wars 2 you're going to have to go on a similar journey for each constituent part of the recipe. But, also like your gaming rig, the neon light spewing machine you'll create after your travels is well worth it.
Just check them out in the trailer beneath.
ArenaNet have published a full blog post about the process of building legendaries over here, but the gist is that they're not telling us how to make them. Well, not exactly. ArenaNet aren't explicitly saying how to make any of the weapons, but they've told us the process and have left hints all over the game's world to help us along.
That process then: a legendary can't be made without a base weapon, these are rare and powerful weapons in their own right, but can be augmented into something vastly more powerful. You'll need to then present a series of gifts to Zommoros, a denizen of Lion's Arch. Then, after Zommoros is laden with items, you'll need to go about collecting reagents like wood, ore, and the like, as well as receiving Karma. All of these are then combined in the Mystic Forge.
Sounds like a bit of a trek, luckily threads on the forums are showing how people are getting along: er, not that well it would seem.