There are various options

A:

Option 1:

Cast on 20 stitches
Rows 1-2: Knit
Row 3: Knit 2 together across the row (10 stitches)
Rows 4-5: Knit
Row 6: Knit 2 together across the row (5 stitches) 

Leave a long tail and thread through the remaining stitches.  Pull tight into a circle and secure. Stitch the 2 edges neatly together and stitch into the centre of your poppy. 

Option 2:

Cast on 18 stitches
Knit a row
Leave a long tail and thread through the remaining stitches.  Pull tight into a circle and secure. Stitch the 2 edges neatly together and stitch into the centre of your poppy. 

Option 3:
Sew a white button in the centre of your poppy.