Execution of a legal document requires that the person signing have sufficient mental "capacity" to understand the implications of the document.  Interestingly, the law has standards about capacity and the standards depend on the function that the person is engaging in.

One side of the capacity equation involves the client's abilities, which may change from day to day (or even during the day), depending on the course of the illness, fatigue and the effects of medication. On the other side, greater understanding is required for some legal activities than for others. For instance, the capacity required for entering into a contract is higher than that required to execute a will.

To execute a Will, some courts have eloquently stated that capacity requires ability on the part of the testator to understand and carry in mind, in a general way, the nature and situation of her property and her relations to those persons who would naturally have some claim to her assets. The testator must understand that she is making a Will to distribute her property at her death. As long as the testator understood what she signed when she signed the Will, a later mental illness will not change the capacity at the time the Will was Executed.

The determination of legal capacity mixes medical, psychological and legal judgments. It must be made by the attorney or judge based on information gleaned by the attorney in interactions with the client, from other sources such as family members and other sources if necessary.

Because you need a third party to assess capacity and because you need to be certain that the formal legal requirements are followed, it can be risky to prepare and execute legal documents on your own without representation by an attorney.