Maron Dutton has done a small booklet on Classic (People) Portrait Painting. Although her book is oil-paint based, it shows some useful techniques. Here is a link to her People Portrait book. Here is a link to one of her videos on the subject of Underpainting a Portrait. It’s in oils, so you need to apply acrylis techniques
Melanie Gilstrap (AKA Melanie B) has produced a video that shows you a way of Blending Skin Colors. Here is another of Melanie B’s Blending Skin Colors videos.
Painting By Numbers: The Basics and Beyond is my new book on advanced painting by numbers techniques. Click here to go to your local Amazon web page (link now fixed) and read a sample, and buy a Kindle version (for a Kindle device or Kindle App) or a paperback version. If you are in Australia, New Zealand or Japan; you can buy the paperback HERE.