Henna How To
Henna for Hair
Start with a curved line
There are lots of other ways to do leaves!
Sometimes it is nice to start with the center vein, then do the outer edges, then go in and add detail.
Victoria Welch's Henna Basics: Step by Step is a fantastic book for both beginners looking to learn all they can about the basics as well as professionals who may get stuck in a rut doing the same couple of leaf variations *all* the time.
You can watch some examples of henna leaves being done in these videos:
The first video shows leaves EXACTLY as described above: