I ❤ ribbon embroidery . . . I used 4 mm. ribbon, & it is best that the ribbon being used has a slight stiffness to it. This will help when forming the petals. When Embroidering such small flowers, it is just as important to work the ribbon stitches around a centre as you would with a wider ribbon. The position the needle is brought up for each petal, will determine the way each will form. Also, slide the needle under the ribbon each time to flatten out your ribbon as much as possible.