Observing the Form
The eye is drawn to the center of the flower, so it is tempting to start drawing from the center too. Judging the small distances and overlapped forms in the center can be tricky, resulting in a mis-shapen bloom.Instead, start drawing from the outside of the flower, and look at the overall proportions. Use negative space (examples shown in red) to help you draw the outer petals.

Sensitive Lines
Gradually work in, following the line of each petal as far as it goes. Draw the edges of petals quite firmly, lifting off for a lighter line where the middle of a petal curves away from you. Be ready to trail off a line, lifting the pencil and commencing a new line where an overlapping petal begins.
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Sketching
Pencil and pen drawings of flowers work well when drawn smoothly and cleanly, but also work well when drawn in a more relaxed fashion. Exaggerate the lineweight, using both light and heavy pressure, and loose, even scribbly marks to suggest overlapping petals and foliage.
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Draw Between the Petals
When sketching with line, try drawing the shadows under petals, rather than outlining every petal precisely. Short, stippled marks suggest the stamens at the center of the bloom. Notice that some petals will be foreshortened - some looking longer and some shorter - when the flower is at an angle to the viewer.
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