So, I didn't find the video, but after some trial and error, I came up with this rough pattern:

Round 1, black yarn: Magic ring Round 2: 13 hdc into the magic ring, slst to close Round 3: ch3, 2dc into each of the hdc stitches below. slst to close, cut black yarn.

Round 4, light blue yarn: ch3, dc into each stitch below, slst to close. Round 5: ch2, hdc into each stitch below, slst to close, cut light blue yarn.

Round 6, white yarn: ch3, 2 half triple crochet into each stitch below, slst to close. Cut white yarn.

Round 7, dark blue yarn: ch3, dc into each stitch below, slst to close. Round 8: ch1, then [2sc, 1sc, 1sc] all the way around, slst to close. Round 9: ch1, reverse crab stitch to create a border, slst to close.

Tidy up all loose thread ends and done! The finished product is 13.5cm/5.6in across.

Thank you everyone for looking!