Using 4 mm needles and double knit wool
Cast on 28sts in green or blue
Change to a suitable coloured fluffy / feathery yarn and knit 4 rows
Change to a colour of your choice for the top of the hat, I used a grey yarn, with a satin thread running through it.
Starting with a knit row, stocking stitch 12 rows
(K2tog) rpt to end (14sts)
(P2tog) rpt to end (7sts)
B & T
Sew on the mermaid button using matching cotton
Have a look here for the buttons
Finally, oversew the row ends and turn inside out.