expression, godan verb (-gu)
spin into yarn
Literally means to spin fibers into yarn or thread. Often used figuratively to describe turning raw materials or ideas into a coherent whole.
祖母は羊毛を糸に紡ぐのが上手だ。
My grandmother is skilled at spinning wool into yarn.
綿を糸に紡いだ。
I spun cotton into yarn.
紡ぐ alone means 'to spin' (thread, yarn, or a tale). 糸に紡ぐ explicitly includes the result (into yarn/thread) and is more specific.
Compound of 糸 (thread) + に (particle) + 紡ぐ (to spin). The exact historical derivation is uncertain, but the expression is straightforwardly compositional.