Seer's headband 2 or better (recommended if using Seer's village
|Inputs (128,800)||Outputs (481,600)|
1,400 × Flax (128,800)
1,400 × Bowstring (481,600)
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.