How do you sign a document to make it “legal”?

Unfortunately, there is no uniformity on this. For example, currently executed deeds in Ohio have to be notarized but not witnessed, wills have to be witnessed but not notarized, living wills have to be either witnessed or notarized, contracts generally do not have to be witnessed or notarized (although in many instances it is preferable to do one or both of these things) etc., etc., etc. There are a number of reasons for this confusion. Perhaps the largest one is this: when a state legislature gets together and passes a law that includes language on how to “execute” a specific kind of legal document, it generally is dealing with only that kind of legal document and is not reviewing the requirements it has set up in prior years for executing other kinds of documents. So, from year to year, most legislatures don’t do a thorough review of their respective execution requirements, and these things become difficult to deal with.
It is even the situation that some kinds of documents which require oaths actually require different kinds of oaths from other documents. An oath that is typically required for a living will in Ohio requires acknowledging that the witnesses have not ever been administrators in nursing homes where the declarant has received care, for example. There is no such requirement for a real estate deed. That is just one set of examples.
We welcome the opportunity to talk about these technical issues with our clients, and are happy to help our clients wade through the complexities involved in signing legal papers appropriately.