1
I'm sorry, I misunderstood you.
Start by signing 'sorry' and pointing to yourself for 'I'. Then, use the twisting 'V' handshape at your forehead for 'misunderstood', and finish by pointing to the person you are speaking to.
2
He misunderstood the teacher's instructions.
Point to the person for 'he', sign 'misunderstood' at your forehead, and then sign 'teacher' and 'instructions' to complete the thought.
3
We completely misunderstood each other.
Sign 'we', followed by 'misunderstood'. You can add an exaggerated confused facial expression to emphasize the word 'completely' before signing 'each other'.