All income regardless of it's source would be taxed at the same fixed percentage for all persons residing in the US. There would be no deductions or refunds under this system. The IRS would not need to audit deductions and charitable write-off they would focus on people not paying their fair share by getting payed under the table or by bartering.
If a flat tax system were to be inacted I promise you the wealthy of this country will quickly find ways to reduce government spending and reduce unemployment. Why Because in doing so they will lower the percentage that all of us will be paying. The percentage would be adjusted anually based on expenditures.