SgtMike88Ret has described an excellent technique.
My only suggestion to this is try different angles while holding the striker to see which works best.
With some of the fire steels, they get a blue ish oxidised coating over the surface, and some need a couple of scrapes to get though this coating before they start sparking well.
These fire steels are an alloy manufactured to by softer than the steel used as the striker, so the fire steel wares away creating the sparks. The alloy is also designed to give off very hot sparks. If the striker is to soft, the edge rounds off and the sparks die off. They prefer a sharp edge to work against. Different brands of fire steels have a varying degree of hardness and temperature of sparks.
I make sure that my knifes have an edge on the back of the blade by filing a section so they work with my fire steels. I even filed an edge into my Swiss Tool bottle opener as well because Victorinox round all of their edges too much.
After filing they get a quick test, then left so I dont ware the edge.
I've been told that glass can also be used as the striker but I'm not in any hurry to test it.

When showing a friend that didnt beleive they could make a fire, I used a sheet of paper placed flat on the ground and the shower of sparks ignited it 2nd scrape. Then came the comment "oh, I got to get me one of those"

Another bit of possible useless info: You can start a fire with steel and flint rock. It is harder to do because it works by the flint being harder than the steel, so the steel wares away. If you try this you will find that it needs to much effort to be reliable.