Canon 189. §1 For a resignation to be valid, whether it requires acceptance or not, it must be made to the authority which is competent to provide for the office in question, and it must be made either in writing, or orally before two witnesses.

§2 The authority is not to accept a resignation which is not based on a just and proportionate reason.

§3 A resignation which requires acceptance has no force unless it is accepted within three months. One which does not require acceptance takes effect when the person resigning communicates it in accordance with the law.
[NB see m.p. Learn to take your leave, 12.II.2018, art. 5 (cf. can. 401)]

§4 Until a resignation takes effect, it can be revoked by the person resigning. Once it has taken effect, it cannot be revoked, but the person who resigned can obtain the office on the basis of another title.

§1. Renuntiatio, ut valeat, sive acceptatione eget sive non, auctoritati fieri debet cui provisio ad officium de quo agitur pertinet, et quidem scripto vel oretenus coram duobus testibus.

§2. Auctoritas renuntiationem iusta et proportionata causa non innixam ne acceptet.

§3. Renuntiatio quae acceptatione indiget, nisi intra tres menses acceptetur, omni vi caret; quae acceptatione non indiget effectum sortitur communicatione renuntiantis ad normam iuris facta.

§4. Renuntiatio, quamdiu effectum sortita non fuerit, a renuntiante revocari potest; effectu secuto revocari nequit, sed qui renuntiavit, officium alio ex titulo consequi potest.