General Method Description

Smelting steel bars is a good way of making money for low level players, and also provides slow but steady Smithing XP as well. Unlike skills such as Mining, you smelt at "full speed" as soon as you reach the level you need, so this is a viable option even at level 30 Smithing. At higher levels it continues to be a slow but steady method of earning money and Smithing XP. Note that another option is to combine smelting bars with making cannonballs.

You should complete at least the easy tasks of the Varrock Achievement Diary to get Varrock armour (any level), which gives you access to the furnace in Edgeville, which is the best in the game now that Jagex removed its annoying door. It is right next to the bank, and when wearing the armour you have about a 10% chance of smelting two bars at once. This doesn't make a huge difference with steel bars but every bit helps.

A beast of burden provides no advantages with this method, so at mid levels and above I recommend a foraging familiar such as a magpie, which will provide you a nice bonus in foraged items while you work.