فَإِذَا أَتَيْتُمْ أَرْضَكُمْ
When you reach your land.
فَاكْسِرُوا ... وَانْضَحُوا ... وَاتَّخِذُوهَا
Then carry out the specified actions, sprinkle the water, and make it a mosque.
إِذَا introduces a time condition; أَتَيْتُمْ is past in form but refers here to a future arrival. It is followed by imperative verbs.
إِدَاوَة is a water vessel, and بَيْعَة refers in this account to the delegation’s former worship site.