Begin with your interpreter from the previous laboratory. It should pass all tests. I highly recommend your interpreter makes use of type-case for both interp and subst, as it keeps the code simpler than if you are working with cond.
This may be a good opportunity to talk with some of your other classmates and refactor your interpreter. That is, if you want to clean up your code and make it ready for this lab, and you want to talk to others about what they did, that would not be a bad idea.