Original: Someone saw the unidentified flying object last night.
Passive: The unidentified flying object was seen last night. (We usually omit "by someone" as it's not important who saw it.)
Original: You are supposed to send your application form to the university early.
Passive: Your application form is supposed to be sent to the university early.
Original: They found their room being disorded arrangement.
Passive: Their room was found to be in disarray/disordered. (It's more natural to rephrase this slightly. The original phrasing is already somewhat passive in meaning.)
Original: The war destroyed all the ancient constructions.
Passive: All the ancient constructions were destroyed by the war.
Original: The government will rebuild the Memorial Monument.
Passive: The Memorial Monument will be rebuilt by the government.
Original: The students may elect Bella their leader.
Passive: Bella may be elected their leader by the students. (Or: Bella may be elected as their leader by the students. The "as" is often used in this type of sentence.)
Original: Thu's younger sister dropped her plate of food onto the floor.
Passive: Her plate of food was dropped onto the floor by Thu's younger sister.