Short answer. A joint manifestation under oath, signed by both of you, filed in the same legal separation proceeding. The Family Code asks for nothing more elaborate, but it does have to be joint, sworn, and filed — telling the court informally that you have reconciled is not enough.
What the law says
If the spouses should reconcile, a corresponding joint manifestation under oath duly signed by them shall be filed with the court in the same proceeding for legal separation.
Family Code, Article 65 — Reconciliation: Joint Manifestation. Read the full provision →
Each word in the requirement is doing something
The document must be a joint manifestation under oath duly signed by them. Joint, so one spouse cannot report the reconciliation alone; both signatures are needed, which is the court's assurance that the reconciliation is real and mutual. Under oath, so it is sworn rather than merely stated. And it goes in the same proceeding for legal separation — the existing case, not a new petition and not a separate action. There is no fresh filing to start and no other case to open.
What the filing does to the case
Reconciliation ends the litigation. Where the case is still pending it is terminated at whatever stage it has reached, and where a decree of legal separation has already been issued, the decree is set aside. That is why the manifestation matters even to spouses who have quietly gone back to living together — until it is filed, the case continues on the court's records and a decree that has issued continues to operate, whatever the reality at home.
Property does not automatically go back
This is the part most couples do not expect. Setting aside the decree does not undo everything that was done under it. The separation of property already effected, and any forfeiture of the guilty spouse's share, subsist unless the spouses agree to revive their former property regime. So a reconciliation that says nothing about property leaves you married but with your property held separately. If you want the earlier regime back, that has to be agreed and dealt with — it is not a consequence of reconciling.
Do it deliberately, not casually
Because of that property tail, the sensible order is to talk to counsel about the property position first and file the manifestation knowing what it will and will not restore. Do not sign a joint document under oath about a reconciliation that has not actually happened, or that you feel pressed into; it is a sworn statement in a live case. And if the reconciliation later fails, the acts you forgave may be treated as condoned, which affects any future petition built on them.