We put our handkerchiefs on the grass and sat on them so we wouldn't get our clothes dirty.
この場合、次のような言い方ができますよ。
ーWe put our handkerchiefs on the grass and sat on them so we wouldn't get our clothes dirty.
「私たちは服が汚れないよう、芝生の上にハンカチを敷いてその上に座った」
to put one's handkerchiefs on the grass で「芝生の上にハンカチを敷く」
例:
The grass was wet, but some people just laid down on it without worrying about getting their clothes dirty, but we all put our handkerchiefs on the grass and sat on them so we wouldn't get stains on our clothes.
「芝生が濡れていたが、服が汚れるのを気にせず横たわっている人たちがいた。でも私たちは服が汚れないよう、芝生の上にハンカチを敷いてその上に座った」
ご参考まで!