How do I create advanced reports ?

The first thing you are asked to set to your needs is the product data you want to analyse. Below the "choose product" heading, you will find a drop down menu covering all three AdSense products: 
adsense for content 
adsense for search 
refferals 

Once you have choosen the product, you will need to define a date range. You can either use the drop down menu to define a period of time like today, yesterday, las 7 days... or define your own customised date range like from Sept 5 2006 to Oct 31 2006 to see how the Septembre update affected your performances. 

Next you will need to tackle the "Show data by" section. I understand this is quite confusion so I advise you to choose the default criterion which is "Page". However, advanced adsense users must be able to use any additional data provided by google so let's take a look at how the adsense team explains it. I have modified the text to make it a bit clearer. 

Advanced reports provide the option to view your performance by page, ad unit, or individual ad. We've highlighted the differences between each report below. 
Page reports will show an impression every time a user views a page displaying Google ads. We'll only report one page impression no matter how many Google ads are displayed on a page. 
Ad unit reports will show an impression every time a user views a Google ad unit on your page. For example, if a page with one half banner and one vertical banner is viewed once, we'll display two ad unit impressions. 
Individual ad reports will show an impression for each individual ad that's shown in any ad unit (remembre that a single ad unit can contain more than one idividual ad). For example, if a page with one half banner and one vertical banner is viewed once, it will generate three ad impressions. 

The last decision you need to make is choosing between displaying "aggregate data" and "channel data". 
Unless you want a summary of your overall performance on a daily basis or that you have not defined any channels, I think you should opt for "channel data" option that will give you valuable data on the performance of every channel you set up earlier. 
Once you choose "channel data", more options are displayed. This is a little confusing at first but if you are familiar with SQL queries and databases you shouldn't have any difficulty using these options. For others I have tried to list some guidelines: 
select at least one channel you want to analyse 
set grouping to "date" or "channel", avoid using both 
try many combinations till you get what you want 
try to understand the data and you will see what other data you need