How to make pistachio shells stick together for decorations?
In a nutshell, the answer is your old friend the hot glue gun.

Pretty much any time you need to glue things that are of odd shapes to get them to stick onto other things that are odd shapes you need a glue that takes up space and bonds well (well, I guess "bonds well" is up for debate but... I don't know you'd be expecting these things to be something you can pass down through the generations)... that pretty much means hot glue.

This is supported in a tutorial here and here, which walk you through the entire process. Also, a YouTube video here.