To make a diamond, place 1 diamond block in the 3x3 crafting grid. When making a diamond, it is important that the diamond block is placed in the exact pattern.
Usually, Diamond is found under layer 12, but now if you get to these -Y levels, the odds of finding diamond significantly increase: 50, -64, -59, and -58.
To use Diamond Finder, you'll need to enter your player's X and Z coordinates, the seed of your Minecraft world, and the version of Minecraft your world was.