Idaho Code § 15-5-202
A parent of a minor may appoint a guardian of an unmarried minor by will, subject to the right of the minor under section 15-5-203, Idaho Code. The termination of parental rights of a parent as to the minor shall also terminate the right of that parent to appoint a guardian for the minor. A testamentary appointment becomes effective upon the filing of the guardian's acceptance in the court in which the will is probated, if, at the decedent's death, no parent of the minor was alive who had a right to appoint a guardian for the minor. This state recognizes a testamentary appointment effected by the guardian's acceptance under a will probated in another state which is the testator's domicile. Written notice of acceptance of the appointment must be given by the guardian to the minor and to the person having his custody, or if none, his care, or if none, to his nearest adult relation immediately upon acceptance of appointment. The parent may appoint by will one (1) or more alternate guardians, in order of priority. If a guardian appointed by will fails to accept guardianship within thirty (30) days after the will is probated, or files a notice of declination to accept appointment prior to the running of the thirty (30) day period, or is deceased, or ceases to act after acceptance, then the alternate guardian next in priority becomes the appointed guardian and may file a written notice of acceptance in the court in which the will is probated.
Idaho Code § 15-5-202