A Little History
The classic C shaped fire steel is the most common style of flint striker. Examples can be found from early Roman times (1st-3rd century), through Medieval and Viking eras, up through the 1800's. Some examples are very simple, having a straight taper on each end which is then curled round into the C shape. Some have small, tight curls on the ends. The ends can vary from almost touching each other, to barely looping back a full 180 degrees.
The Romans appear to have had three primary styles: the C style with many minor variations, the P or R style, and the Sled or Sleigh style. Examples of these styles can be found from at 1st to 3rd century archeological sites on up through 16th to 19th century Persian pieces. The P and Sleigh styles appear to have gone out of common use and manufacture in the 14th and 15th centuries. Other styles include: a straight tapered rod in a wood handle (like an awl); U or horseshoe shaped with ends cut at an angle out from between the two legs; D shaped straight striking bar with one end tapered and looped back; and a C variant with a straight striking surface with both ends tapered and looped up to twist together to form a triangle.

Using a Firestarter and Flint
How do you take a chunk of steel, hit it with a rock, and get a fire? This is amazing the first time you see it! But, it does take a little knowledge, some proper tools, and some preparation.

The fire steel needs to be high carbon tool steel.
A piece of flint works best, but any hard rock with a sharp edge will work. Flint just works better, and holds a sharp edge longer. It's also easier to knap the edge to sharpen it.

Then you need something to catch the sparks. Charred cotton or linen cloth works very well (be sure the cloth is flammable). When a spark lands on it, it catches, and the glow will spread throughout the whole chunk of cloth as you gently blow on it. Once you have that spark in your charcloth, you then need a "bird's nest" of very dry grass, or bark, or old rope. And then all the small twigs and kindling to build your fire.

When you strike a glancing blow with the face of the fire steel across the sharp edge of the flint, you get sparks. What you are doing is cutting or scraping little bits of the steel off with the sharp edge of the flint. Similar to pushing a piece of steel into the bench grinder and getting sparks. It takes a little practice, and you have to be careful that you don't hit your knuckles on the sharp flint!

You strike your flint and steel so that the sparks land on the charcloth. When one catches, you place the charcloth in your "birds nest" of dry bark/grass. Gently wrap the birdsnest around the charcloth and spark. Now gently blow on it. As the spark spreads out through the charcloth, the heat is transferred to the birdsnest. When it gets hot enough, you have FIRE! You then place the burning 'birds nest' into your prepared kindling, and build your fire.