How is an irrevocable divorce effected after the husband has divorced his wife with a single revocable divorce and then taken her back?

The husband's taking back of his wife after the first divorce is valid before the expiration of her waiting period (iddah). If the husband desires an irrevocable divorce (Talaq Ba'in), then either he divorces her twice, in which case she becomes unlawful for him until she marries another husband, or he divorces her once and refrains from taking her back until the iddah expires. In the latter case, a new marriage contract with all its pillars is necessary for them to reunite. It is worth noting that divorcing more than once or issuing consecutive divorces before the expiration of the iddah is not from the Sunnah.