For the beautiful Snowflake Hydra, I used cotton thread size 10 and a 2 mm crochet hook. The finished motif is 10,5 cm (about 4 inches) in diameter. You can be sure it will look amazing in a variety of colors and yarn weights. Besides the yarn and hook, you will also need a needle to weave in loose ends and a pair of scissors.
If you want to stiffen and block the snowflakes you will also need white school glue, stainless steel pins (or toothpicks), plastic wrap and some kind of foam/blocking board.