Speak to any doctors who cared for the testator around the time the will was written. Ask the doctors if the testator was mentally sound at the time. The testator's mental capacity after the will was written is irrelevant, so you must focus on doctors who cared for the testator around the time he drafted the will. Ask the doctors for written statements describing the testator's mental capacity around the time the will was made.
Speak to other people who cared for, or interacted with, the testator when the will was made. These people might include friends, family members and other medical personnel. Ask if the testator was mentally competent when the will was made and if he understood what a will was. Ask each person if the testator understood how much property he owned and who his legal heirs were. Ask each person if there were any events that reflected the testator's mental capacity. Ask for written statements from each person describing the testator's mental capacity and any related events that happened around the time the will was made.
Get copies of the testator's medical records. While having a condition that affects mental competency isn't enough to prove a person didn't have the mental capacity to make a will, the presence of some conditions, such as Alzheimer's, may add weight to your case.
Read the private correspondence of the testator around the time the will was made, if possible. Statements the testator made may show a lack of mental capacity. For example, if you find a letter that shows the testator didn't understand the will he made or wasn't able to identify his heirs, it supports your claim he lacked mental capacity.
Make copies of all the evidence, including statements from the doctors and witnesses, as well as any private correspondence that supports your case. Keep the originals together in a safe place.
Go to the court holding the probate proceedings. Ask for the forms necessary to contest a will. Forms vary by state. You will need to complete and file a petition and motion in most states. Attach the evidence copies to the petition. File the forms in court.
Notify all interested parties in writing of the will contest and first hearing date and time, using the method the probate court directs you to use. You may have to mail or have the notice personally served by an adult not related to the case; the notice may be a copy of the petition or motion you filed or both. Ask the clerk for a copy of the court rules if you're unsure about the service method. Interested parties include the estate executor, other heirs and will beneficiaries, and any attorneys involved.
Speak to all the persons from whom you took statements. Tell each person the date, place and time of the hearing. You may ask them to testify at the hearing, or inform them they may be asked to testify at subsequent hearings.
Attend the hearing. Bring the original statements and evidence to the hearing.