Collecting (mining, fishing,woodcutting) is RS gold obviously a simple and strong way to earn gold. Runecrafting on the other hand needs additional skill/investment but appears to be the most profitable way to make money in Runescape. Assuming you already have the membership and you are looking for Gold to extend it, you should be able to perform just fine with any of the ways listed here. It doesn’t require you to manipulate the market entirely, but you require knowledge about items’ pricing and actual values when doing this. Additionally understanding when particular items may become more vaulable is a good point to start (such as when you are good enough to understand patch notes and such).
Largely depends upon the skill. Among the fastest skills for attaining 99 throughout the board (regardless of whether you are a regular player or a flavour of Ironman) is Firemaking, as Wintertodt has made it rather easy to realize high XP prices. In cases like this, you can expect to go from 1 to 99 Firemaking in roughly two to three weeks, depending on how much you play and if you currently have the prerequisites for efficient Wintertodt beyond 50 Firemaking (1–50 needs to be done the normal way, though this is still quick ).
But if we consider buyables, Construction is arguably the fastest skill to train in the game – should you’ve got a couple billion coins burning on your pocket, at the least. With a reasonable method, like mahogany planks, 1-99 may take about 3 weeks, but by using the very costly marble cubes and magical stones a single week could theoretically be enough (even though it’s so ridiculously expensive nobody might look at doing it other than as a joke or bragging rights).
Farming deserves a mention too, as once you unlock kronos crops you can get 1M XP daily just doing the daily tree/herb runs, and even farther by RuneScape Mobile gold using tithe farm. Depending on how quickly one can find the amount required for kronos, a week or two might be for the speediest, non-buyable skill (Farming is buyable), I’d say Thieving is most likely the fastest. May take about a month to get to 99 out of 1, but it has no real requirements and you can’t really use money to gain a big advantage.