You have two things.
1. low carbon steel
2. A lump o' coal.
How do you get the lump o' coal into the low carbon steel to make high carbon steel?
Well, you can and you can't. If you want a bar of high carbon steel that is hi carb all the way thru then buy it. If you want to add carbon, you can, but only on the surface, this is called Carburizing. Steel is made up of atoms of iron and carbon that hook together. When you heat it, the atoms of iron vibrate faster and move further apart. When this happens, carbon can be added (around 1800 degrees F). I use Barium Coke in a box. This process takes time, (4 to 8 hours) and you are only getting around 40 thousandths depth (.040) of carbon. There are many ways to add carbon, this is only one. I only use this when someone brings me a frizzen that doesn't give the spark they want. Carburizing is also good for shafts that have a tough core and a hard surface.