Using flint and steel to light a fire is somewhat difficult. In order to use a flint and steel, you take a hard, sharp-edged rock in one hand and the steel striker in the other. The "flint" can be any hard, sharp rock, such as flint, jasper, or quartz. The striker can be any piece of high-carbon steel, such as a knife blade, though strikers made specifically for this purpose work much better.
The other essential item needed for starting a fire with flint and steel is a char cloth. A char cloth is a piece of partially burned cotton cloth. It can be made by cutting pieces of cotton into 1-inch squares, placing them in small a metal container (such as an Altoids tin), and placing that in a fire. If the cotton cannot get much oxygen, it will char rather than burn. Remove the tin from the fire when it quits smoking.
Before you begin, lay your fire so that when you ignite the tinder with the flint and steel, you can place it in the fire and get it going. Once the tinder is lit, it's too late to lay the fire, as the tinder will burn for only half a minute or so.
Place the char cloth on the flint even with a sharp edge, but not covering it. Hold the striker loosely in the right hand (assuming you are right-handed) and strike it against the flint (held in the other hand) as if you were trying to shave the striker with the rock. Flint is harder than steel, so it will indeed shave a piece of steel off the striker. When this piece of red-hot steel is shaved from the striker, it will fly off and hopefully land on the char cloth. You may need to repeat this several times before a spark does actually land on the char cloth, but when it does, the char cloth will catch the spark and begin to glow itself. You then take the glowing char cloth and set it against your tinder. Raw cotton and oakum work especially well as tinder. Blow on it gently until you get a flame. If you don't blow hard enough, the ember will not ignite the tinder. If you blow too hard, you will burn through the tinder without igniting it. Once it flames up, place it in the fire you have already laid.
