RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Unstrung Magic Longbow
The Old School RuneScape Wiki also has an article on Unstrung Magic Longbow.

An unstrung Magic Longbow can be made by using a knife with some magic logs requiring level 85 Fletching, giving the player Fletching 91.5 experience. It can then be used to make a Magic Longbow by attaching a bow string to the unstrung Magic longbow, giving the player 91.5 Fletching experience, giving a total of 183 Fletching experience.