Adoptions, whether adopting a non-biological child or a step-child, require District Court hearings. Documents must be filed with the Clerk of District Courts office located in the Law and Justice Center in order to make the matter legal.
The Clerk's office does not have templates to assist you in filing the proper documents. However, the Law Library of Montana website does contain documents for Step-Parent Adoptions here.
The filing fee for an adoption is $105.