Accidentally posted this draft from a while ago yesterday ... Since it's here: I participated in the Sktchy app (only available on Apple devices, but they have a FB page and you can join challenges and get the reference photos via email) 30 Faces/30 Days challenge in January, when I was living with my mom. To keep it simple (and even so I only made it about halfway through the month!) I used only a mechanical pencil.
I found it very engrossing!