Note: It's important to never get too attached to an underdrawing and be able to wipe it out and completely rework it at any time, including during the sitting if it loses spontaneity or doesn't reflect the experience of real life.
I began with a grey toned 18x24 canvas and built a sketch of about 8-10 lines to position her in space and capture her head tilt. Then, I focused on the exterior shapes of the head/hair and used indicators to place the eyes, nose and muzzle area around the mouth. From there, I worked from big shapes first. As the angles and position of big shapes in the drawing began to make sense together, I moved down to the next level of detail iteratively. Finally landing on the forms of the eyes and working on the details of the face to finish the underdrawing.
Now that the foundation drawing was ready, I felt inspired and was excited about the upcoming sitting. I had decided in advance that to accomplish the look we wanted for this painting, I'd invest the time in the step of completing on a semi-closed grasaille (a painting in just grey to focus on the values and volume of the head shapes) and then scumble the color on top.
This photo from the beginning of the sitting shows where I've started to establish the highlights as place markers and other 3D shapes of the face. This step is helps with the smile by making sure the shadows cast by the folded muscles don't flatten the shapes. The goal is to keep that rounded feeling. Next post ... finishing the grasaille and enjoying the subtle color of the skin - particularly the rosy cheeks.