In Texas, someone can make a Will on their own, but it must be in the person's own handwriting and show "testamentary intent," which means they intended the document to be their last will and testament.
Texas Estates Code 251.052 allows that a Will, written wholly in the testamentor's handwriting, is not required to be attested by subscribing witnesses.
This means that if the decedent wrote out a Will then as long as it's in their own handwriting, and there's someone to testify in court to the authenticity of that handwriting, then that handwritten document can likely be used as a Will in a Texas Probate Court.
Caution should be used in using this as an Estate planing technique. A Will is a specific, special document that needs to be tailored to meet your needs. Consultation with a Wills and Estates attorney is advisable before undertaking such a task on your own.