I based this work on a photo I found in this interview with Boris Karloff's daughter, Sara Karloff: http://crypticrock.com/interview-sara-karloff-reflections-on-boris-karloff/
Correction:
As it turns out, what I thought was a photo is actually the work of Rick Baker. Thanks to Mark Robinson and Dave Robinson for pointing this out. So I am hereby giving credit to Rick Baker for the awesome work that I mistook for a photograph!
The process started with this preliminary sketch
Added detail to the sketch
More shading and darker details
Added highlights
I used the Adobe Photoshop Sketch app with an Apple Pencil on iPad Pro
Added background - Final