I suggest that you start by checking that students are familiar with the third conditional, if you haven’t done this already.
You can then move onto mixed conditionals. Find a text you can use to present mixed conditionals or show students sentences such as the following:
If I hadn’t gone shopping, I wouldn’t have found those gorgeous boots.
If I hadn’t gone shopping, I’d have enough money to go out tonight.
Ask students to look at the phrases and to discuss in pairs a) which tenses are used in each sentence and b) what is the difference in meaning in the two sentences.
The first sentence refers to two past events whereas the second refers to a past event and a present result.
As follow-up practice, you can prepare sentences using third conditionals and mixed conditionals. Cut up slips of paper: some should contain the beginning of the conditional sentence and other papers the second half of the conditional. In pairs or groups, students can put together the beginnings and ends of sentences to produce as many correct sentences as possible.
All the best,