Although there are so many digital fur brushes available, it's very hard to get a satisfying result with them. It's because they're tricky—they won't paint fur for you if you don't know how fur works. In fact, they make it even harder!
In this tutorial I'll show you how to draw natural-looking long fur with only one, quite universal brush. Once you get it, you'll be able to use other brushes to accelerate the process, too!
Warning: this tutorial is about painting fur, not about painting a lion or drawing anything. You need some basic knowledge of digital painting to make use of it!
1. Sketch a Lion
To present the process I'm going to paint an imaginary large cat with very thick, bear-like fur. By adding a lion's mane to it, I'll be able to show you two kinds of fur.
Step 1
Start by sketching a few different poses on a very small scale. Use an irregular, preferably slightly textured brush for it. Don't draw any details, and don't zoom in—the smaller the pose, the easier it will be for you to keep the proportions right.
Step 2
Choose one of the poses and copy it to a new file. Resize it with the Free Transform Tool (Control-T). Fill the background with 50% gray (
#808080)—it's the most neutral background you can get. 
Step 3
Lower the Opacity of the pose to 20%. Create a New Layer and sketch the lion using the pose as a guide. It doesn't need to be that refined at once; take your time and create more transitional steps if needed.
Step 4
Sketch the main "hair masses". If you don't like something about the pose, this is the time to fix it.
Step 5
Discard the pose layer and lower the Opacity of the other one. Create a New Layer. Now you can sketch some more refined line art, although it's not necessary. It just may be helpful to sketch your vision and compare it to what you want to achieve.
2. Create the Base for the Painting
Step 1
Discard the previous layers, leaving only the line art (or the sketch). Create a New Layer. Pick a hard brush and draw an outline of the lion using a vivid color. It doesn't need to be 100% accurate, but try to predict the outline of fur.
Step 2
Use the Magic Wand Tool (W) to select the area outside of the outline. Invert selection (Control-Shift-I). Create a New Layer and fill the selection with the Paint Bucket Tool (G). You can now remove the previous outline.
Step 3
Press Control-U and lower the Saturation of the shape to make it more neutral. This will be the base for our painting. If you create a New Layer over it and clip it (Control-Alt-G), nothing you draw on it will go outside the outline. Clip the line art to the base, and then paint the details that have nothing to do with hair. Keep them on a separate layer all the time.
Step 4
Create a New Layer between the base and the line art, but below the details. Fill it with very dark gray, but not black.
Step 5
Create a New Layer. Sketch the direction and length of hair on the whole body. It's always good to find a reference of an animal similar to ours and try to see the flow of fur on its body.
When done, invert the colors (Control-I) and set the Blend Mode to Overlay.
3. Paint Flat Colors
Step 1
You can use any textured brush you wish for this part, but if you want to achieve an identical result, feel free to download "texturedbrush.abr" from the tutorial attachments.
Create a New Layer. Paint the flat colors very roughly. I've decided to give my lion a dark mane and dark underside to make it more original—it's more common for animals to have a lighter underside (useful for countershading camouflage).
Step 2
Create a New Layer. Pick a color of ambient light (in my case it's the classic blue of the sky) and paint roughly over the areas in the shadow.
Step 3
Set its Blend Mode to Multiply. If the effect is too strong, lower the Opacity; if it's too weak, use Control-U to lower the Lightness.
Step 4
Create a New Layer. Pick a color of the main light source, and paint over the illuminated areas.
Step 5
Set its Blend Mode to Screen, and lower the Opacity to your needs.
4. Shade the Fur
Step 1
Time for the most important part! Pay special attention to what's going on in every step—don't just copy what you see, but try to understand the process instead. This way you'll be able to use it in the future, for every animal, in every pose.
Create a New Layer over the "fur flow" layer (you can create a new one for every step in this section, or, if you're more confident, paint them all on one layer). Pick the color of the shadowed area with the Eyedropper Tool (I).
Now we need to blend the shadow with the midtone, but not in the usual way. Draw the areas between hair clumps with that darker shade, but only on the border between light and shadow. The deeper you go into the light area, the more subtle and less uniform the shadow should be.
Step 2
Now pick the color of the midtone and paint the clumps between the light and shadow areas.
Step 3
Finally, pick the color of the area in the light and paint the clumps here, subtly coming into the midtone area.
Step 4
In some places fur "breaks", creating characteristic "cracks". Draw them with a dark, but saturated shade.
Step 5
Pick a lighter color from the nearby area and subtly enclose the "cracks".
Step 6
Back to the whole body. Pick the color of the illuminated area and make the brush much smaller. Increase the Brightness and paint very thin, slightly chaotic hairs in the light only.
Step 7
Pick the color of the shadowed area, make it a bit darker if needed, and stress the darkest crevices with a small stroke. Paint over the dark areas, occasionally crossing the light clumps with hair-like lines.
Step 8
During the shading I've lost my original coloring, so here I've added a New Layer inMultiply mode and darkened the dark parts.
Step 9
Pick a color from the shadow. Increase its Brightness, but decrease the Saturation. Paint the clumps on the opposite side than the highlights—this will be reflected light. Don't cross all the shadow with them!
5. Render the Details
Step 1
Create a New Layer on the top, but don't clip it this time. By picking the colors right from the edge, draw single hairs and small fur clumps all around. You can use a different brush for it—some of the fur brushes will work very well here!
Step 2
Merge (Control-E) all the layers (except the background). This merged layer will be our new Clipping Mask.
It's time to draw the tiniest hairs, but only in the illuminated area. Start with the mane. Pick the color from its light part, make it brighter, decrease the size of your brush, and draw the individual hairs.
A point to remember: animals in the wild rarely have long, smooth fur. The longer it is, the greater the chance that it's wavy and coarse. Therefore, don't draw these hairs as straight lines—shake your hand a little and make them cross each other in an unpredictable way.
Step 3
Do the same with the body, using shorter lines.
Step 4
You can come back to the mane once again and add a few brighter hairs. The fewer of them you add, the better they will look.
Step 5
Add these bright hairs to the head, too. When it comes to the nasal bridge and the fur around the eyes, feel free to use a small, thick fur brush. If you decide not to, you can get an even nicer effect by drawing these hairs manually.
Step 6
Our lion is definitely painted, but it still needs a final touch. When we paint something with a smooth surface, it's very easy to add big lighting. Here we need to be more careful, because big strokes will flatten the fur texture.
Create a New Layer. Pick a color from the light on the mane and make it much brighter, more light-source colored. Paint over the area that has the best access to the light source. Remember not to overdo this effect—it doesn't fit every part equally.
Step 7
To blend it better, double click the layer. Go to the Blend If section, hold Alt, and drag the black marker on the second slider. The farther you drag it, the more transparent the layer will get over the dark part of the fur.
Step 8
You can use this method to add more layers of bright fur. If you want to blend them better, use Overlay mode.
Step 9
Add very bright hairs to the rest of the body, using the mane as a reference for this light scheme.
Step 10
Reflected light should be darker than the main light, but it still should be visible if we want to achieve a nice 3D form. Pick a color from that area, make it brighter and less saturated, and stress some of the hairs. Don't use big strokes!
Step 11
Because fur works like a thick material, subsurface scattering can be used here. This phenomenon makes the dark parts more saturated when light comes through them. To simulate this, go into Quick Mask Mode (Q) and paint over the border area between light and shadow. It doesn't work everywhere, so be careful!
Step 12
Exit the mode and invert the selection (Control-Shift-I). Open Window > Adjustments and select Hue/Saturation. Drag the Saturation slider to the right.
Step 13
Once again, I've added a Multiply layer to darken the darker parts of the fur. This is also a good time to merge (Control-E) all the body layers and use Filter > Liquify to fix the overall shape with the Forward Warp Tool.
Step 14
Create a New Layer. You can use the Soft Round brush to paint over the edge to create a subtle back light effect.
Step 15
Finish the picture. If it looks too dark for you, you can play with adjustments:Brightness/Contrast, Levels, and Curves. You can also select some areas andFilter > Sharpen them.
We're Done!
In this tutorial you've learned a set of rules for creating natural-looking fur. Feel free to use them to invent your own technique—you're not supposed to copy me, but rather use this tutorial as a support for your own development. Also, don't forget to study the fur of various animals to paint it accurately—the technique itself is worthless when you don't know what you're painting!
