Since most of my subjects are animals I am going to go into a bit more details on how I scratch short fur to create a realisitic look to it.
Steps 1-3 are done with a #11 craft knife or scalpel (depending how fine you want your lines to be).
1. I start out with light weight lines that are nearly parallel in structure. These lines go with the direction of the fur and will create the foundation for the fur direction. Following fur direction is very important to help give the correct shape to the animal and realism to the piece.
2. Then I add hairs that are out of alignment and cross over the parallel hairs, some straight and some with slight curve or curl at the end. Since fur does not stay in perfect alignment this gives it a more natural look. The longer the hair the more it will cross. Short smooth hair will be close to parallel (only deviating maybe 3-5 degrees from the other hairs), while long hair will have much more crossing (many different angles).
3. Adding another layer of hairs with slightly more x-acto/scalpel pressure to make somewhat heavier lines if the area is a light value, once again following the general fur direction, but deviating some as well. (Note if leaving an area darker I don't do steps 3 or 4)
4. Occasionally, if the hair is coarse, I go back with the pointed speedball tip and add thicker hairs over the thinner lines.
Steps 1-3 are done all very quickly and I work the overlapping lines all at the same time rather than doing it in actual separate layers, as shown here. If done enough it becomes very natural and simple. The above is much larger than in real life as well, just to show more clearly how I work. In realty each of these areas would be very small.
When done over the whole animal it gives a natural 3-D look with tonal variation, fur direction and a depth that is very realistic.