Here is my feeble attempt at explaining my process of creating the Agent Coulson caricature. You should realize that I don't always do it this way. There really is no wrong or right way to do it. It just seemed easier to do it this way for this one. Here goes.
1. I start with a sketch done in Photoshop. You can do the sketch on paper and scan it in also. Doesn't matter.
2. Next, I duplicate that layer then fill the bottom layer with white. The new line drawing (layer 2) is change to multipy mode. Then on the bottom layer I lay in my colors. Also use this layer to get a feel for the tone and lighting.
3. Next, I create a third layer on top of the other layers. On this layer I start to block in colors, lighting and shading. It's important to not fall in love with the line drawing. The lines are just a guide. So don't worry if the colors you lay down don't match the lines. A lot of changes can happen during a painting and it's important to be able to realize and adapt without worrying about you lines at this stage. Also I tend to use a fairly large brush at this stage. No need for details here.
4. Up until now I've been working at 100 dpi. That's because if I work at 300 dpi, my computer tends to lag. But now I up rez to 300 dpi and get into all the details. This can be the longest part of any painting. Details. Parts 1, 2, and 3 took about 1 hr. Part 4 takes a little over twice that long. Sometimes it can take up to 8hrs to finish a painting just because of all the details. It does require a little patience. There really are no short cuts in painting the facial details of a caricature so you just gotta buckle down with your favorite songs, food, drinks or tv show and get it done.