I have been taking a few pictures as I went along, to show you how I do this. I am sure there are many other ways, but this works for me and as far as I can tell, is the traditional method.
Materials used here are my stitching thread (for this small piece I used Gutermann hand quilting thread, 100% cotton. It is quite strong and fine. I usually use Dual Duty handquilting thread with a polyester core, which is very strong). You can see the white tape I am using for finishing.
I am pinning the edge of the tape to the last line of stitching, working from the front. You can see that the tape is facing downwards, so I can flip it up and over to the back when the first line of stitching is complete.
The tape will stand up a bit from the work. Be sure to follow the original stitching line (I stitch exactly on top of it)
Here is the"dangerous" part !! You have to trim the excess fabric away. I do this from the front so that I do not accidentally cut into the tape, and I keep the work on a flat surface. I trim to about 1/4", and I clip the inner curve 3 times - once in the middle, and then one clip either side of that, about 1/8" apart. Do not clip too close to the stitching.
Just for comparison: this is an old petasson, reverse side. You can see how this one is finished with the tape.
I will post a final picture later when I have stitched down the final seam. The next part will be to wash: start with a soak in cold water for several hours to remove any remaining marks and help close any needle holes from stuffing, then wash in hot water with a mild detergent or Marseille soap. Then lay out flat to dry on an absorbent towel. I turn mine over once the top feels dryer, so that it dries evenly. Sometimes I will pin to ensure the piece dries straight.
I use a frixion marker to trace the design, which I remove by running a hairdryer over the piece before washing. Better than ironing - remember a boutis must never be ironed. I use this method for my applique too, it avoids the annoying white mark that these markers can leave.
Any questions ?