Practice piping the petals on a plate before you move to the cookie. It takes a little practice.
Start with plain buttercream or two-toned buttercream. Use tip 104. Have the skinny end of the tip facing away from you and the wide and towards you at the base of the cookie. The petals of the apple blossom are skinny and get wide as they go out. Start squeezing in the center of the flower cookie with your bag angled slightly down, start piping slowly, pull upward to the tip of the petal. Round the petal by making a slight rounding motion with your wrist. Pull down once you’ve rounded the petal and slowly stop squeezing when you reach the center.
Repeat this technique pulling up, rounding and pulling down with each petal.
As you pull down get as tight as you can to the base of the petal so it stays skinny.
Take tip 366 and leaf green buttercream. Have the beak facing down and at the end of the petal. Start squeezing with a large amount of pressure until that buttercream billows out and slowly pull back. Stop squeezing where you want the leaf to end and the pull away to break the buttercream.
Mix a tiny bit of green and yellow food coloring gel to your plain buttercream to get a pale-yellow green. Attach tip 233 for the stamen in the center of the blossom. Hold the bag straight up and down and squeeze for a second and slowly pull straight up and stop squeezing, pull away to break the buttercream.