1
Oh shit, I forgot my keys!
In this context, the sign is used as an exclamation of frustration. Sign 'shit' with a sudden, sharp downward pull of the thumb to emphasize your annoyance, followed by the signs for 'forget' and 'keys'. Ensure your facial expression matches the sudden realization.
2
That movie was shit.
Here, the sign is used as an adjective to describe poor quality. Sign 'movie', point to your reference space, and then sign 'shit'. Pair the downward pulling motion of the thumb with a disgusted or disappointed facial expression to convey your opinion clearly.
3
I stepped in dog shit.
This uses the literal meaning of the word. Sign 'dog', then sign 'shit' using the standard thumb-dropping motion from the bottom of your non-dominant fist. Follow this with the sign for 'step-in' and a grossed-out facial expression.