Well you know how to layer shades properly and thats the hardest part (imo) but I find that its best to either do observations indoors with controlled light if your going to spend allot of time on it, the lights color changes allot during the day. I kinda cheat and use what i know to help while Im working since I have trouble actually seing some colors as what they really are. (Colors appear to be different based on their surroundings) Dont start with skin though, do something smooth and flat so you can focus on the color and not the shapes

good luck mate ^^