You need medium-weight (DK or worsted) yarn and a 5mm crochet hook. The pattern uses single crochet (sc) and magic rings. Gauge: 14sc × 16 rows = 10cm square.
Make a magic ring, chain 1, then work 8 single crochet into the ring. Pull the tail to close. Place a stitch marker at the start of each round.
Round 2: 2sc in each stitch (16 total). Round 3: *1sc, 2sc in next* repeat (24). Round 4: *2sc, 2sc in next* (32). Round 5: *3sc, 2sc in next* (40). Round 6: *4sc, 2sc in next* (48). Continue increasing every round until the circle measures 15cm across.
Continue in the round without increasing for approximately 15cm — this forms the sides of the hat. Try it on as you go — the hat should sit comfortably on the head with a bit of stretch.
For the last 4cm, switch to a slip stitch rib: work in back loops only for a stretchy brim effect. Fasten off and weave in all ends with a yarn needle.