You can use any yarn weight, just make sure to use appropriate hook size. Pictures show snowflakes that are blocked and stiffened using water-based glue. If you want to stiffen and block your snowflakes there is also a simple tutorial on my website.

This pattern uses US terminology and these abbreviations:

sc – single crochet
ch – chain
slst – slip stitch
hdc – half double crochet
dc – double crochet
tr – triple crochet
ML – magic loop/circle/ring