Its something like this:
You buy from the item mall (yeah i learn it

).
You add to your inventory
Open the alchemy, put the item/weapon/jewl on it and add the stone of astral
Voila you have Astral in your weapon
(Same as you do in Elixirs or Stones of tablets)
One more thing, the Astral prevents your weapon to reset to +0 when doing the "tunning" process. Of course if you have Astral (1times) it only prevents the first time reseting!
Hope i give you a little help on that.