I secretly borrowed the stepladder that was in the garden without asking, then quietly returned it when I had finished with it.
「脚立をこそっと借りとこ」= 「secretly borrow a stepladder without asking」
ボキャブラリー
脚立 = stepladder
こそっと~する = secretly, sneakily
借りる = borrow
無許可 = without permission/without asking
I secretly borrowed the stepladder that was in the garden without asking, then quietly returned it when I had finished with it.
(庭に置いてあった脚立を無許可で借りて、目的が達成された後で、こっそりと返した。)