Enter the text id of an item (for example stone or iron_sword) into the appropriate slots. You can enter an arbitrary item for the head or hand, but the other slots must contain an appropriate item.
In 1.13, you can now add formats to the name tags. Enter the color id (for example red, dark_blue) in the name color field to give it a color.
This thing uses WebGL! If you don't see anything on the left enable WebGL in your browser or visit the official WebGL website to troubleshoot problems.