Adding the facial features on top of the face. You don't need underlay when stitching on top of an existing object, however an edge walk underlay will help keep the edges of a satin stitch object nice and crisp.
You will find Part 1 of this series here. https://youtu.be/0fiskrUYdbk
and part 2 here. https://youtu.be/3-DNLfgMOpE
Part 4 can be found here. https://youtu.be/38XnC40VfvQ
0:00 Intro
0:50 How to see the image under your stitching.
1:50 The elipse tool in detail.
8:08 Mirror Merge the eyes.
9:45 The nose and mouth.
13:55 The eyelashes using the Travel Tool Bar