A detailed design is simply the next layer in the design process, taking up where the overall design and architecture leaves off.  It includes a class diagram for all module classes that includes properties and methods, plus pseudo-code for all methods.  This pseudo-code will vary in detail, from super simple for trivial methods to fairly complex for larger methods.  It's important to understand the purpose of the design review, which is to look at the detailed design and flow, thus five lines of pseudo-code does not realistically represent a 500 (or even 50) line method. In particular, pseudo-code should cover all decision making, flow control, looping, and error handling in a method or function.

Interaction or flow diagrams may also be necessary, showing how the classes interact with each other and how errors are handled (especially how they are unwound).

If an MVC architecture is being used, this is where the detailed interactions are worked out, especially the exact controller operation, as this is where some major abstraction mistakes can be made.  In addition, the model details are outlined at this level, both to settle on the abstraction models and to simply lay out the model algorithms.  View must be delineated along with all of their possible gestures.