To start, you bring your needle up through the fabric
and then back down, forming a simple stitch. I made this stitch a little long so it is the long stitch of "long and short" stitch:
Now come back up through the fabric next to your long stitch
and take a smaller "short" stitch!
Now come back up
and make another long stitch
You just continue doing this in stitches of varying lengths
until you fill in the shape! According to 100 Stitches, this is an excellent filling stitch and is sometimes referred to as a form of the satin stitch.
100 Stitches also mentioned that this is a good stitch to use to achieve a shaded effect which is what I have done on the sunflower petals: