Seer's headband 2 or better (recommended if using Seer's village
|Inputs (249,200)||Outputs (593,600)|
1,400 × Flax (249,200)
1,400 × Bowstring (593,600)
The best possible method is to use the spinning wheel directly underneath the Lumbridge Castle bank. A full load of 28 flax takes approximately 50 seconds to convert into bowstring. Travel and banking time is ~20 seconds in between loads. You should use a preset with flax in your inventory to speed up the process.
An alternative method is to wear Seer's headband 2 and run between the bank and Seer's spinning wheel house, above the ladder, and you will be able to spin the flax 33% faster.