This is one of the most rewarding things about learning about the outdoors. To make what has caused man to evolve from a piece of shit on the ground to a piece of shit on the ground that can talk.
It's much easier than it sounds, but does take practice. You need a piece of flint rock, a piece of steel large enough to hold and strike the flint, and some very fine fiber material for starting. The ideal material would be charred lint (lint that has been pre-burned) because it catches sparks well and keeps them hot. Some dismantled rope makes an excellent "nest" for the char cloth. The nest catches to actually make a flame (with proper blowing). Nests are ideally made of frayed (not nylon) rope.
The sparking process is the most complicated part. You must first make your nest my placing a 1 inch to 1.5 inch round piece of char cloth (once again, char cloth is ideal) on a nest. Pull the nest up around the sides of the char cloth just enough to cradle it. Firmly hold the flint right above the nest. Quickly and sharply strike a sharp edge of the flint with the steel. If you do this right, it should spark. It takes a bit of practice to get the angle right. Now spark the flint until you get some good sparks to catch on the char cloth. If you know your char cloth is good and caught, pick up the nest carefully, and pull some of the frayed rope over the char cloth and gently blow. If done perfectly (and there is no other way to do this part), the little ember of the char cloth will produce enough heat to catch the rest of the nest on fire. If the nest catches, you're done. Now place the nest into the previously set up fire pit and proceed to cook marshmallows.