It depends on what you mean by fast:
The fastest is just basically buying your bars then smithing the highest level thing you can at an anvil.
The way I did it was to mine mithril/coal and super heat bars at either the guild or lumbridge swamp mine. Bank these bars and smith later for high alchemizing.
Another way is to mine gold ores then smith them with the Goldsmithing Gauntlets from the family crest quest.
Using the blast furnace isn't too bad its actually pretty fast.
If you're looking to train smithing without mining ores then make cannon balls or you can also buy ores, superheat/smelt and then sell the bars to make a profit and decent exp.
Those are the main methods I can think of at the moment
