undeclared income from previous years

Can anybody help. A clients wife has recently told me that she has undeclared income from previous years. As she was not my client I told her to contact HMRC who have sent her a 'Notice to complete tax return' for 2011/12. I have now filed a return for 2011/12. Her income consists of a salary from her husband which is taxed under PAYE but she has other self employed income. What is the best way to declare her self employed income for the previous 3 years. I believe there was a recent question on this but I cannot find it. Any help would be appreciated.