Beautiful little hat, Pauline! 
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To keep the sides tighter make sure you have a long enough cable (I use 40" for hats). I also try let the loop at the end kind of cross or hold it closed with my little fingers. That keeps it neater.
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To make decreases even make sure you have a number that can be divided evenly. For my standard hat I cast on 80 so I can decrease once in each group of 10. Like this -
~ knit 8, k2tog (place marker) around hat
~ knit even
~ knit 7, k2tog (slip marker) around hat
~ knit even
~ etc…
- If you need to use a number other than a multiple of 10 figure out whether you can decrease evenly before you start. It’s easier.
