。
She showed them how to fasten their seatbelts.
Literal
She [topic-は] seat-purpose [genitive-の] belt [genitive-の] fasten-method [object-を] them [to-に] taught-politely.
Two productive Japanese affixes light up this sentence. The suffix ~用 ('for use with X / intended for X') turns 座席 ('seat') into 座席用 ('for seats' — a seat belt's purpose); the same ~用 gives 子供用 (for children), 業務用 (for business). The verb-stem + 方 frame builds 締め方 ('way of fastening') from 締める ('to fasten') — productive for any verb: 食べ方 (way of eating), 書き方 (way of writing). Stack those together with the genitive のs (座席用のベルトの締め方) and you get a tightly nested 'the fastening-method of the for-seat-use belt.'