The new basecamp has a very nicely designed instructions for adding discussions and todo lists via email. For anyone who has tried to design a feature like this, it's always a challenge to describe the multiple steps and make it easy to remember. There are two things from this design that really stand out.
First is It describes the action items in a clear numbered-list. Use links and bolded fonts to highlight the important points.
The second is the "conceptual" preview of how an email will be converted into a todo list.