You must now buy coal and iron to start melting. (Make sure that you purchase at least 2x as much iron in your coal, e.g. 15 iron, 30 coal). It is not necessary to make them. Just sell them on the Grand Exchange (G.E.). Today you can get 914 grams for every steel bar that you sell at the G.E. It is possible to stop melting steel bars once you have reached 1M gp.
If you want to know more about RSorder, you can go to https://www.rsorder.com/