Gauge is not important.

If you use a yarn or thread that corresponds to a 3.75 mm hook, your cover will fit a 2.5 in diameter ornament. Using worsted weight and a 5.0mm (H) hook will make the cover fit a 3.5 in diameter ornament.

For my cover I used a double length of crochet thread and the 3.75 mm hook.

Other materials and tools:
2.5 in diameter ornament
tapestry needle
scissors

Video Tutorial:
https://www.youtube.com/watch?v=P2N138pqfcw