From Academy Bylaws:
Article III, Section 1. Membership shall be by invitation of the Board of Governors. Invitations to active membership shall be limited to those persons active in the motion picture arts and sciences, or credited with screen achievements, or who have otherwise achieved distinction in the motion picture arts and sciences and who, in the opinion of the Board, are qualified for membership.
To be considered for invitation to membership in the Designers Branch of the Academy, an individual must:
(a) have at least five years experience in the motion picture industry, and have screen credits* as Production Designer, Art Director or Set Decorator on at least four feature films of a caliber which, in the opinion of the executive committee, reflect the high standards of the Academy. For Art Directors working under the supervision of a Production Designer a minimum of six screen credits is required.
(b) have screen credit* on a picture nominated for the Production Design Award.
(c) have, in the judgment of the Designers Branch Executive Committee, otherwise achieved unique distinction, earned special merit or made an outstanding contribution within his or her field of endeavors.
Paragraphs (a) and (b) shall not pertain to those crafts within the branch which do not receive screen credits.
Proposals must be accompanied by a letter from each sponsor which addresses, as specifically as possible, how the candidate meets one or another of the requirements above.