It is just like making magic item and then you add the legendary.
So you make a +1 long sword as normal. When a magic item ascends to legendary status, it gains the suite of base legendary abilities and can have up to three additional legendary abilities.
Hope that clears things up.
I've got a campaign that's on hiatus for a while, but the Paladin in the group who'd bonded with Radiance had been "talking to it" in character, as if it had been awake... Then it talked back, after they picked up Legendary weapon feature (which is still needed for the player to activate abilities). The features chosen were:
Flexible Bond (allowing other party members to work with Radiance)
Upgradable (allowing for adding enchantments to Radiance, see next entry)
Intelligence (Radiance became fully intelligent, adding the rules for Intelligent weapons. The Upgradable feature allows new abilities to be added, without effecting the base values of a Holy Avenger, but effects the Ego of Radiance.)
An additional side effect of gaining the Intelligence feature was that one special ability on the list could be chosen (while others could be bought or upgraded with additional selections of Legendary Weapon)I felt the "Transfoming"ability might be interesting.
So, end result is that Radiance now becomes a little fairy-like creature (the size or mass of the weapon), who can work with anyone in the party, and can be upgraded with an 8-hour ritual and currency. Intelligent item abilities are easier and cheaper, since I use the full value of Radiance in her (yes, HER) fully activated state for calculating magic weapon adds.
Hope this helps. And for how we've been playing this... See the Anime "Soul Eater" for reference... It's been some fun sessions.