This is by far the most annoying crafting achievement simply because of how the game handles upgrading and ascending weapons. You need to get any regular weapon to +15. This will take a lot of patience and you can't get this until late game.
Required Embers and locations
Large Ember - Found in the first room of the Depths. First, grab the basement key on the other side of the gate in the Undead Parish, then go back to the beginning of the bridge guarded by the Hellkite Dragon. Then kill the Capra Demon boss in the Lower Undead Burg. This will give you the key to the Depths. Step back out of the boss fog once defeated and look to the left. Follow the path down and open the door. The ember is located in a chest to the left of the butcher's table.
Very Large Ember - Found in a chest in the end of New Londo Ruins. AFTER YOU'VE OPENED THE FLOOD GATES (get the key to the seal from Ingward) go down to the bottom of the area. There should be a well with a corpse containing six red eye orbs. Go into the building and beware of darkwraiths. The first mass of souls enemy can be found in this building. Go past him and go into the room to the right. Follow the path up the stairs to get to the chest with the ember.
You need a lot of titanite for this. In the end, you'll need 9 titanite shards, 9 large titanite shards, 7 titanite chunks, and one slab. Slabs and chunks can be obtained using the methods described in the 'Lightning Weapon' achievement and the various titanite shards can be bought by the Giant Blacksmith in Anor Londo.