Start by writing two sentences on the board; one correct, one incorrect. Ask the students to find the mistake and correct it. Explain that you are going to give them each a worksheet. Half the sentences are correct and half are incorrect. Divide the class into two groups, give half the students Worksheet A and the other half Worksheet B. Allow the students some time to correct the sentences.
You'll find that some students will make mistakes at this stage by correcting the already correct sentences and marking the incorrect sentences as correct. When students have finished correcting their sentences, explain that they are going to work with a partner to find the correct sentence. Tell students that their partner has a similiar worksheet. Together they have all the correct answers, but if the sentence on Worksheet A is correct then it is incorrect on Worksheet B and vice versa.
This is quite difficult to explain so it might be worth going over the first pair of sentences together as a class so that they get the idea. You want the activity to be as communicative as possible, hopefully students will read their sentence carefully to their partner and explain why it is right or wrong. This can lead to students discussing what they think the grammar rule is. What you want to avoid is students simply swapping papers and ticking off the sentences. It is a good idea to physically separate students by placing them in two rows of chairs a short distance apart.
Once students have chosen what they believe is the correct sentence, give students the answer sheet. Wrap up the activity by asking students which question they thought was the most difficult and answering any questions they might have.