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You can optain the flax buying them at the ge or picking the flax by: My flax picking guide http://forums.rsbandb.com/viewtopic.php?f=66&t=72603
To make bowstring from flax requires 10 crafting.
You also need a spinning wheel.
I suggest to use the spinning wheel at the second level lumbridge castle or southwest of the seers village bank.
1.Put 28 flax in your bag.
2.Run to the spinning wheel.
3.When you are at the spinning wheel click on it then right click on the picture of the flax and press "make x" Press 3 or 4 numbers quickly and press enter.
4.Go to the bank and repeat the steps untill you are out of flax or your fingers get sore.
5.Sell your bowstings at the ge or save them and train your fletching level!