For this stem, I started with a couple of couching stitches at the top of the thread, next to the flower:
Using Fiona's design as a guide, I laid the thread on the fabric, and moved it about until I was happy with the line it took. Unfortunately, it wasn't just going to sit there and wait for me to stitch it down - it needed to be held in place. So, I added a handful of stitches all the way down the thread:
Here it is with half a dozen or so stitches down the length of the thread:
This was enough to hold the shape of the line as I went back up the thread, couching it down with stitches spaced about a millimetre apart:
And with a couple more stems added:
For the first time, I actually quite like the fern!