Or how to replace a Steiner/Waldorf doll's embroidery without undoing the wig.
Step 1: Thread a long doll needle with desired colour of embroidery thread and knot the end. Snip close to the knot and pass through from the back to one side of the mouth
Step two: Pull the thread back through and pass underneath the hair and then back through to the front
Step three: continue this step until the mouth is as full as you'd like (normally two or three stands) and then you are ready to finish off. It's important that the knots are secure or the thread will come out through play.
I've included a couple of photos to show you how I do this.
Do this once more and trim the thread and push back under the hair and you're all done