The part that is free to play in this OSRS gold guide is quite simple. To gain a significant increase in Smithing level, you must take on your Knight's Sword quest. When you complete the quest, it will give an individual 12.725 Smithing XP that will allow you to increase your Smithing level from 1 Smithing up to 29. Another option to allow F2P players to earn Smithing XP while earning gold is to make rune2h-style swords. You can create them once you reach level 99 Smithing, but by drinking the flagon of Dwarven Stout, you can increase your Smithing levels by +1 and so begin making Rune 2h swords when you have reached level 98 Smithing.
It is always a great alternative, if not just to gain XP gains, but also to gain other game features that you can unlock. The quests are usually worthwhile to complete but the question is when the right time to complete them. It is worth noting that the Knight's Sword was mentioned in the F2P section of the guide.
P2P players can also access such quests such as Elemental Workshop 1. and 2. It is also possible to play The Giant Dwarf, Heroes' Quest, and it's highly recommended to complete Pirate Pete subquest from Recipe for Disaster.
Complete The Knight's Sword quest alone provides buy RS gold plenty of XP however, we strongly advise to finish The Giant Dwarf or at minimum, finish it part-way for entry into the Blast Mine.