Dark Souls

While ninja277 HAD a great chart layed out to help with understanding how the weapon creation works, for this particular weapon achievement there is an even easier way.

Once you reach the Duke's Archives (far into the game) there will be a chest you uncover which nets you he Enchanted Falchion curved sword. (Sorry i can't remember the exact location of the chest) All you need to do from here is upgrade it to a + 5 and 'POP' achievement is your's!

To upgrade this to a + 5 you will need:
Enchanted Falchion + 1 = 1 Blue Titanite Chunk
Enchanted Falchion + 2 = 1 Blue Titanite Chunk
Enchanted Falchion + 3 = 2 Blue Titanite Chunks
Enchanted Falchion + 4 = 3 Blue Titanite Chunks
Enchanted Falchion + 5 = 1 Blue Titanite Slab

You can upgrade this to a + 5 yourself at a bonfire (if you have the smithing boxes purchased) or any Blacksmith

For obtaining the items needed to actually upgrade the weapon at all here's what i found:

-- By this late stage in the game i had already enough Blue Titanite Chunks to upgrade it to a + 3. Chunks can be found within the Duke's Archives off Dead Bodies and in the Crystal Cave's where you face Sheath the Scaleless (Big Dragon)

-- If you need to farm Chunks, the Blue Golems drops them (though they are rare) and are found at the bottom of Dark Root Basin where you fight the Hydra and Free Dusk the Merchant from the Golden Golem.

-- More Blue Golems can be found in outside the entrance of the Crystal Caves (10 or more) in the little garden area, if you travel all the way around following the castle walls you'll engage many. This spot is a great place to farm if you need them being RIGHT next to a bonfire.

-- The Crystal Caves themselves have several Golem's, though not too many. Also when traveling the Seath's domain you will wander through a tiny cave after dropping down ontop of an invisible platform, several of the Crystal Lizards are there which can net you some if you kill them fast enough

-- As for the Blue Titanite Slab itself, it is also found in the Crystal Caves, shortly at the beginning while descending down the slopes you can take a small invisible path to the left to where a dead body lies holding the item.

It would be also wise to note, unless you are going to be personally using the weapon for something, gather all the necessary materials required to upgrade and do it IN ONE GO. Once you do, and the achievement pops DASHBOARD IT! Reload the save and you will still have the Slab which is only one per game. I myself did this and kept my slab to use for another upgrade later

To create a Enchanted Weapon you need to begin with a standard weapon ( one that you can appply pine resin to otherwise it is unique and will not work )

Then you will need to recover Enchanted Ember which is located in the Darkroot Garden (This is located in a chest in the middle of a very large Puddle of water in an area with the tall and small mushroom men walking around) and give this item to Rickert of Vinheim who is located in New Londo Ruins. This allows you access to the Enchanted branch of weapons which he can then upgrade to +5 Enchanted.

In order to complete the Enchanted weapon from scratch you will ultimately require 9 Titanite shards , 9 Large Green Titanite shards , 8 Blue Titanite Chunks and 1 Blue Titanite Slab

Blue Titanite Chunks-Crystal Golems and Crystal Lizards (recommended to wear fog ring against lizards) inside the Crystal Cave, However the best place to farm chunks of all kinds is at the end of the game int he Kiln of the First Flame ...all black knights have a 100% chunk drop rate and are respawnable.

BlueTitanite Slab - Can be found in Crystal Cave by traversing the invisible walkway after the Crystal Lizards(See vid for actual location of walkways, the key is to watch where the snowflakes fall because they only fall over the walkways). Bring lots of Prism Stone.

Here is a Chart which was created by an unknown so credit to whoever they are.

TRICK-To get the achievement without using your items simply fully upgrade the item in one shot and wait until the achievement pops WITHOUT leaving the upgrade window thus not allowing the game to save the data then simply turn off the game and turn back on.

Although in the stock game you can only find one Blue Titanite Slab in a single play through, if you own the Artorias of the Abyss DLC there is also another you can find.

The extra Blue Titanite Slab can be found in the Royal Wood just outside of the Oolacile Sanctuary. It is located approximately in the middle of the first area after you cross the bridge. There will be a small body of water guarded by 2 smaller enemies and a Stone Guardian. To the right of these enemies there is a treasure box hidden in the grass which contains the Slab.