How to implement cascading parameters in a SQL Services Reporting Services MDX Report

Last week, I needed to prototype a report against an Analysis Services 2005 cube for a client.  The report wasn't too complicated... I just had the requirement to allow a user to drill down a hierarchy by selecting a member at one level, then displaying all the children for that member, etc.  This is really easy to do in SSRS... but I wouldn't go so far as to call it super intuitive.  It had been awhile since I wrote a MDX report, and it took me a few minutes to remember just how to do it... so I thought I'd document the steps, for my own benefit if no one else's.

The sample requires the Adventure Works AS DB.  You can download the sample here.

Scenario

So, I want to create a report that returns the Order Count by Product.  The users need to select a particular Category, and then Subcategory for the products they want to see.  The report also needs to return summary information for both the Category and Subcategory level, and it needs to contain the Category and Subcategory on the report.  This can be a pretty standard business case for reports where the potential data set is to large to report on.