Fishbone stitch is great for filling spaces, especially for making leaves.
To begin, push the needle through the fabric at roughly the centre point of the leaf shape from the back to the front. Pull the thread through, leaving a tail to secure into the work at the back as you continue to sew (as shown on the 'getting started' page). Push the needle back through the fabric from front to back a short distance straight down the centre of the leaf, as shown in the picture below. Pull the thread through.
Bring the needle back up through the fabric from the reverse side to the front slightly below and to the side of your first stitch point, following the line of the leaf shape.
Push the needle back through the fabric slightly below and to the other side of the bottom of the first stitch, so that your second stitch overlaps the bottom of the first stitch, as shown below. Pull the thread through.
Repeat the action above for the next stitch, taking the third stitch in the opposite direction so that it overlaps the bottom of the first two stitches.
Continue to work your way down the sides of the leaf, alternating sides so that each stitch overlaps the bottom of the previous stitch.
The stitches should increase in length as you get to the middle of the leaf and then get shorter as you reach the bottom. This gives the leaf it's shape.
For a very neat finish you should aim to keep your stitches lying close to each other to prevent any gaps (I have noticed a small gap on mine near the bottom!)