This stitch has technical similarities to the fishbone stitch and the opened fishbone stitch. The difference is that this stitch cannot give sharp ends, and therefore, is not  ideal for leaves with tapering ends. Instead, this is one of  the best stitches to fill in flower petals.

I will be working on a petal shaped pattern to demonstrate. The pattern has been divided into four by two lines inside. The lines are called A,B,C and D.

flat_stitch_1     flat_stitch_2
Fig 1: Bring the needle out from the line A and put it in through line C. Give it a slight slant, but not too much.   Fig 2: Bring the needle out through line D and put it in through the line B. Again, bring out the needle through A and continue the procedure. Remember to keep the stitches close to each other.
flat_stitch_3   flat_stitch_4
Fig 3: The trick for a neat pattern is to keep the points on A and D parallel with each other. Similarly, keep the points on B and C parallel to each other. Half way through, our pattern will look like this.   Fig 4: The completed pattern will look like this.