All you have to do to begin it is level 10 Mining so requirements are extremely minimal and completing this quest will definitely be quicker than RS gold other training methods. In addition, if you're already somewhat experienced player and have reached the level of 20 in Magic, level 20 in Mining and level 20 in Crafting you are able to complete both parts of the Elemental Workshop quest.
This should take you straight into the 35th level without worrying about any other Smithing leveling methods. If you want to continue in the quest, you may complete the Giant Dwarf quest which later is required for other methods. There are additional Recipe for Disaster, Devious Minds, Cabin Fever, Elemental Workshop II, Family Crest, Fremennik Trials that you can finish for the fastest experience. Keep in mind that all of this quests are for members so F2P players aren't able to complete them. Unfortunately, neither free to play quest is rewarding players with Smithing levels.
LEVELS 29-40 IRON ITEMS. In this bracket, you'll want to make the best possible iron objects from iron bars that you can create - such as Iron Platebodies. Achieving a level of 40 should take around an hour with about 1000 bars of cheap OSRS gold input. As previously mentioned, the ideal place to do this is at the western Varrock therefore start making parts of Armour there.