Postnuptial agreements aren't a big favorite with the courts, and there are pretty strict requirements about how they must be prepared and signed. Even if you draft the document yourselves, make sure you talk to a lawyer about the requirements for your state. If you do write up an agreement, you'll have to do it without benefit of any software -- none is available for this purpose. All the more reason to make sure you find a lawyer to help you before you complete the process.