Short answer. File a written opposition in the probate case stating your grounds for opposing the will's allowance, and serve a copy on the petitioner and everyone else interested in the estate. Rule 76, Section 10 requires this of anyone appearing to contest the will; a bare oral objection is not a contest the court can act on.
What the law says
Anyone appearing to contest the will must state in writing his grounds for opposing its allowance, and serve a copy thereof on the petitioner and other parties interested in the estate.
Rule 76, Section 10 — Contestant to file grounds of contest. Read the full provision →
The opposition must be in writing
Rule 76, Section 10 is short and strict: Anyone appearing to contest the will must state in writing his grounds for opposing its allowance. It is not enough to show up at the hearing and object in general terms. The written opposition frames the issues the probate court will actually try — each ground you intend to press should be stated in it, because the contest proceeds on the grounds you have put on paper. It is filed in the same special proceeding in which the petition for allowance of the will is pending, before the will is allowed.
Serve it on everyone interested
The same section requires you to serve a copy thereof on the petitioner and other parties interested in the estate. That means the person who brought the will to court, and the others with a stake in the outcome — the heirs, and those named in the will as devisees or legatees. Service is not a formality to skip: the allowance of a will settles rights in the estate, so everyone whose share turns on the result is entitled to know what the contest is about and to meet it at the hearing.
What kind of grounds belong in a contest
Probate at this stage asks whether the document is really the deceased's last will — whether it was executed with the formalities the law requires, whether the testator had the capacity to make it, and whether the signature and the act were the testator's own, free of fraud, duress or undue influence. Grounds of that kind belong in the opposition. What generally does not belong is a complaint that the shares are unfair: how the property is distributed under a valid will is a different question from whether the will should be allowed at all.
Positioning yourself before the hearing
Be ready to show your interest in the estate — typically that you would inherit if the will were disallowed, or that you are named in it or in an earlier will. Then watch the hearing date fixed by the court and file the written opposition before the will is allowed, serving the copies Section 10 requires. Gather what supports each ground now: the witnesses to the execution, medical records bearing on capacity, specimens of the testator's signature. A lawyer will want to see those before deciding which grounds can honestly be pressed.