The law surrounding wills, trusts, powers of attorney and estate planning dates back centuries. Over this period of time, rules have been established both by law and the courts to prevent fraud that aren't necessairly intuitive. Sometimes, lay people aren't aware of these rule and it creates pitfalls if you try doing it yourself. The Minnesota Court of Appeals recently faced a case where the deseased person, Ester Sullivan, had validly executed a will in 2006. In 2008, she decided to amend the will, made a photocopy of the will and went to work trying to change the terms herself. The results were remarkeably unsuccessful. You can read the decision at the following link: In Re The Estate of Sullivan.

The "take away" from this case is that it's really important to get advice from a professional in the field and not try doing it yourself.