§1. To be valid, a resignation, whether it requires acceptance or not, must be made to the authority to whom it pertains to make provision of the office in question; this must be done either in writing, or orally in the presence of two witnesses.
§2. The authority is not to accept a resignation which is not based on a just and proportionate cause.
§3. A resignation which requires acceptance lacks all force if it is not accepted within three months; one which does not require acceptance takes effect when it has been communicated by the one resigning according to the norm of law.
§4. A resignation can be revoked by the one resigning as long as it has not taken effect; once it has taken effect it cannot be revoked, but the one who resigned can obtain the office by some other title.
Source: Code of Canon Law (Vatican.va)