I made the skulls a few years ago, and attached them to a skirt for a Mexican zombie party. They are made out of one of my grandma's old cross-stitched floral tablecloths. After the party, I unpicked the skirt and used a couple of them for other things, before putting them away in a safe place. Quite typically, I promptly forgot where that safe place was! Luckily, I stumbled over their faux-mouldy-awesomeness again recently (turns out I'd hidden them in a handbag).
To make the collar, I simply hand-stitched the skulls together, before attaching several layers of fusible interfacing to the back. I stitched the interfacing at the edges, to stop any lifting. It joins at the front with hooks and eyes.