2.75-3.25 mm hook. You will need to play to get the right size hook.
Chain 5, make a loop.
Chain 1, SC 10 in loop, slip stitch to join.
Chain 1, *In next st SC 1, HDC 1, DC 1, HDC 1, SC 1 - in the next st SS 1* Repeat around from * to *
They are super simple, I woul try using a light weight yarn though, I used worsted weight (4) and my fingers were cramping afterward, it is just too small of a flower for the heavy yarn
As for the flower center, I do not make those centers but it looks to me like it is just a tight loop of chain sts.
This is just how I make my small flowers, so they may not turn out looking exactly the same.