Completed by then: Futur II
Futur II combines werden with Partizip II plus haben/sein. It can describe completion before a future reference point or a conjecture about a past event. The context tells you which meaning is intended.
Understand the idea
Futur II looks at an event as completed by a future reference point. It can also express an assumption about a completed event, depending on context. It combines the future auxiliary werden with a perfect infinitive.
Build it step by step
Conjugate werden for the subject. Place the lexical participle plus haben or sein at the end of a main clause. The final auxiliary follows the verb's Perfekt pattern. A clear phrase such as bis morgen helps establish the future-completion reading. German may use other tenses when context already makes the timing clear.
See it in a sentence
Bis morgen werde ich den Bericht geschrieben haben.
Bis Freitag wird sie alles bezahlt haben.
A useful memory aid
Build the perfect infinitive first, then place a conjugated werden around the sentence frame. This keeps the two auxiliary jobs separate.
A common trap
Do not choose sein simply because the sentence refers to the future. The final auxiliary still depends on the lexical verb's perfect construction.