For the first time, you can switch your into one of the official Blast Furnace worlds. If you're less than level 60 make sure to bring some gold coins since you'll have to pay the Blast Furnace Foreman from time to time. You'll also require potions for RS gold stamina as you'll get exhausted after several minutes. This method gives amazing experience rate, especially at lower levels but is very costly. For levels ranging from 40-99, it takes about 65mil (after selling bars made).
LEVELS between 74 and 99 ADAMANT TIPS. Another alternative to the Blast Furnace that is expensive is to craft adamant Dart tips from adamantite bars. Level 74 is required for those that make some profit. However, this isn't the only reason to use this method as Dart tip making is AFK friendly. With this method it is possible to make around 60k per hour which is quite decent. If you're considering this method be aware that you'll have to finish The Tourist Trap quest beforehand which helps you create darts.
LEVELS 35 99 Cannonballs. This is one of the oldest and most well-known ways of earning money that require Smithing skill. For this method, you have to complete Dwarf Cannon Quest and bring steel bars with you that are needed to create Cannonballs. By doing this, you can earn the player around 10k exp per hour, and more than 100k gold. Although it is a decent income generator for the lower levels, there are better options in the cheap Runescape gold future, so be sure to switch to well paid ones. Experience gains that are slow are not worth it.