To create a Magic Weapon from scratch 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 acquire the Large Magic Ember which is found in the Duke's Archives and give this to Rickert of Vinheim who will then allow you to upgrade from +5 Magic to +10. Rickert of Vinheim can make Magic weapons to +5 by default.See video for ember location credit to MrSiegebreaker13 for the vid.
In order to complete the Magic weapon you will ultimately require 9 Titanite shards , 10 Large Green Titanite shards , 9 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, fifth Black Knight at the Kiln of the First Flame
BlueTitanite Slab - Can be found in Crystal Cave by traversing the invisible walkway(See Video by deaDEV1IHD NOTE:the key to finding this path in game is watching the snow, it only falls where the ledge is located but use prism stones to verify) after the Crystal Lizards.
Here is a Chart which was created by an unknown so credit to whoever they are.
TRICK-To get the achievement without using your final 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.