The shape of the ellipse of the outer edge of the cone and the location of the flower bottom are the two most important reference points to catch in your preliminary sketch. This fairly stiff demonstration emphasizes the geometry of the shape. Keep these steps in mind when drawing a real flower. This framework will help you capture the geometry of the flower. On top of this you can add the individual character and twists of individual petals (a topic for another blog).
Click on the first image to start a step by step slideshow.
Draw a cone: start with an ellipse and the point of the bottom of the cone. Connect the sides of the ellipse with the bottom point.
Put dots around the edge of the ellipse where your petal tips will end. Stager these slightly so that the front and back petals do not line up precisely.
Draw lines from the dots to the bottom of the cone. These will be the axis lines of the petals.
Now draw front to back, starting with the elements that are structurally closer to you, here the sepals.
Draw the petals on the close side of the flower first. The closer a petal is to the side, the narrower it will be.
Draw the petals on the back side of the flower with less pressure. This will make them appear to be behind the front petals. Note that all the petals point to the bottom of the cone along the axis lines you drew earlier.
Stamen orient the same way, with the filaments connecting to the base of the cone.