Theres 2 ways I know.
1. you could buy flax 100each, then spin it and sell 150-160ea, and then buy more and more flax until you reach your level (It is around 15K xp per hour, I did this from 35-40 craft to 78 and it works well, though it's awfully boring)
2. you could kill green drags and make vambs off them (they require 57 crafting to make them, so youre good with this)
1. you would have to spin 4,546 Bowstrings (lets say around 5 hours)
2. you would have to kill 1,100 Green Drags. Thats WAY too long finally lol... but it could give you around 2.5M selling the bones.
But ya if you really want fast and cheap xp i'd go with flax lol.
If people have better ideas, i'm open too, because i'm so bored of flax
