The other pieces you need to create Tiamat's Tear are the 3 Rank-A Dragon Stone Swords; Dragon Soul, Sigil of Dragon Command, and Bahamut. As you can see in the below picture, it will tell you how many correct swords you need. The below picture shows all 3 swords with a gold border. That means it is ready to be fused now. (The check mark is also a pretty nice hint). If it shows a grey border, that Rank A sword is still needed. Hovering your mouse over a grey bordered sword will tell you what Rank B sword it needs.
Successfully fusing the 4 components together over 48 hours will result in your sword.