This is a step by step on different techniques used in creating my lasted work "Bear Hat Revisited". It covers how to color match, use a reference, use layers, render skin, and rendering hair.
First I will open a reference or several reference to work on for different details in my works that focus on realism. I start with a light sketch on one layer to plan my drawing. Note that I try to scale the reference image to the same size as the canvas to help with matching my proportions even though I stretch the neck and level the shoulders. This sketch will help me plot colors next.
TIP: To help with color matching, drag your color palette as close to the reference as you can and adjust the color to match the base skin tone in your reference. Aim for a medium tone with an even color base. So if half the skin has a red hue and half has a yellow hue, go with a tan hue so it will be easier to alter later on in the drawing.
Now we make a new layer and fill it in with the color we picked for the skin. I use the fill tool and then erase what I don't need. This provides an even background for our shading and highlights and will have clean edges after erasing. You can also erase with a soft edge for a blended effect. This is similar to using masking techniques in photoshop to get clean edges and help with working your image into backgrounds and other layer elements later on.
In order to erase the parts you don't need, move your sketch layer to the top and set it to "multiply" then decrease the opacity to a level where you can see your sketch but it does not block being able to clearly see your skin layer. You can always erase more later so I left some areas like the ear and neck a bit over the lines so I wouldn't have to paint more base skin tone and have more work later on.
Next we repeat this step for other elements. Mainly in this one, the hair.
Here I have repeated the color matching, fill, and erase steps for the new layer for hair. Having these elements on their own layer will help us later when we want to add a background or shade behind the image without getting any overspray from the airbrush.
Note that I used the soft eraser to start blending the fine hairs on the hair line. This is often a tough area for people to get right and then the hair feels like its a wig. The hair has to softly start on the skin and work into the hair color. So being able to use a low opacity eraser on soft will usually do the trick.
TIP: In order to get the skin to feel more realistic it is important to add texture to your piece. I zoom in close to the picture and let it guide me on what spots appear in the photo. Using this as a guide, I set my airbrush to scatter and smooth and max out the opacity. Then I will paint differing skin hues and add white in areas that have highlights for a glistening effect. The greater the contrast in your skin tone to highlights the more the skin will appear "wet" or "shiny". Less contrast will make the skin look like it has makeup or appear flat.
This is what the skin will look like after you have textured it. It is hard to resist adding details at first, but it pays to do this step before adding the eyes and working on sharper details like the nostrils and mouth.
TIP: Try to match your drawing size to the size of your reference as much as possible if you want a close match. This will help you spot issues early on that can consume a lot of time to fix later. Here I use it to make sure that the nose is portioned properly.
Next I will add shadows and details with the airbrush set at a low opacity and reduce the smoothing and scattering to zero.
TIP: Shade skin with colors. Shading with straight black no matter at what opacity will distort your colors and "gray out" your image. On this piece the shadows on the face are actually dark red. Other times you might find that dark blue works well for shadows since cool colors recede more, but this can cause too much depth for areas on the face where a short depth of field is needed.
Now that you have worked the shading of the face and the color of the skin, you can reward yourself by adding some details in the face. Again, even the black in the eyes and under the nostril are really dark red. Only use straight black for the absolute darkest dark in your image.
TIP: In order to get eyes to have more depth, use a low opacity airbrush with pure colors like red, blue, or yellow to softly intensify the base color you used for the iris. Then darken the edges of the iris and then the pupil. This is usually the darkest part of the face, so make sure you paint with a high opacity on your shadows and then touch it up with a high opacity high light or two to make it shine. Some lower level opacity high lights can help add reflections in the eye, but don't over do it as it will make your image look worse from further away i.e. ("bad thumb"). Again you'll see that my reference is in extreme close up mode so that I can see all the little details that make the features work.
Now we move on to the hair.
Hair has always been one of my strengths, which is lucky for me in this drawing. It is important to think of rendering hair like fabric and forget that it is millions of individual strands. Fabric is also millions of threads and like fabric hair will group and bunch in the same areas meaning that shadows and highlights will be connected. Focus on using a high opacity airbrush with a strong highlight color (here it is gold) and a very dark shadow (here I used dark red again). Start by alternating between painting in highlights and shadows working your way around the hair in sections. Remember that you do not need to paint a piece of hair from start to finish. Each section will have its highlights and shadows that will guide you into the next section. It won't look impressive at first, but after a while you can zoom out to see how well it is working.
TIP: Do not use burn and dodge to render hair. This is a mistake because it seems so easy at first, but most people who use this technique will end up with frizzy and blown out colors. Burn and dodge areas after you have rendered to add punch.
Here we focus on the shadows behind her. Working on another layer, add your shadows with a low opacity airbrush(I mean really low opacity 7% to 13% max). Using a dark blue, I softly start laying in the shadows behind her hair starting with the darkest shadows and then working out to the lighter shadows. Doing this prevents having darker shadow overlays. You want each pass to go over the whole area of the last spray so that you get a nice even shadow spreading outwards from the darkest part.
Here I finish the highlights on the neck and add in the shading of the collar bone. Use straight white for shiny skin and be aggressive with those highlights. Often times people won't push the highlights on skin and it results in a flat modeling of the body. For the shadows I go back to using low opacity dark red to keep the pink tone of the skin intact.
Now you have three rendered layers: Hair, Skin, Shadow. Go ahead and delete the sketch layer so it doesn't cause problems if you decide to flatten your image for effects. Now you can add a new layer to go behind all of these layers for your background. I have never been able to work a background first, so working in this method means that I have clean top layers and can play with the setting of my portraits after I am done rendering them. Try filling your background with different base colors at first and see how they effect the portrait. Compliments to red and brown worked well with this one since those are the dominant colors in the foreground.
TIP: To achieve the modeled background in this finished piece use a really big blending brush and vary the shade tint and purity of a color and go wild mixing them up. It is a lot of fun and at one point it will look great and you can stop :)
Thank you for reading my tutorial and I hope it helps you. Sorry about all the text, but I was always more of a verbal instructor than demonstration teacher.