What information should I submit to the IRS?

The most important thing that needs to be submitted to the IRS is a clear concise statement of what the tax underpayment issue is.  It is also very helpful to describe how much tax was underpaid, and to what years the taxes relate, and how you became aware of the issue.  You should absolutely submit to the IRS all the relevant evidence that you have to support your allegation that the taxpayer is not paying the amount of tax they owe.  A tax case is typically built on documents, so to the extent you have them and they support your allegations, they should be shared with the IRS.  If you have no evidence to support your allegations and cannot point the IRS in the right direction to find any such evidence, it is possible that that your submission made a “less than substantial contribution” to the IRS’ efforts in the case and they may cap your award at 10%. 

In summary, you should provide the IRS with information that convinces them that they can make a strong case against the taxpayer for the taxes you allege they have not paid.