Simple Future Tense

Active: Subject + will/shall + first form of the verb + object
Passive: Object of the active sentence + will/shall + be + past participle form of the verb + by + subject of the active sentence

Active: I will write a letter.

Passive: A letter will be written by me.
Active: She will help me.
Passive: I will be helped by her.
Active: John will learn the lesson.
Passive: The lesson will be learnt by John.

Changing a negative sentence into the passive

Active: She will not help us.
Passive: We will not be helped by her.
Active: We will not visit the hill station this year.
Passive: The hill station will not be visited by us this year.
Active: We shall not betray our country.
Passive: Our country shall not be betrayed by us.

Changing an Interrogative sentence into the passive

Active: Will you help him?
Passive: Will he be helped by you?
Active: Will you not help me?
Passive: Shall I not be helped by you?
Active: Will they accept our invitation?
Passive: Will our invitation be accepted by them?

Future perfect tense

Active: Subject + will/shall + have + past participle form of the verb + object
Passive: Object of the active sentence + will/shall + have + been + past participle form of the verb + by + subject of the active sentence

Active: We shall not have accepted the invitation.
Passive: The invitation shall not have been accepted by us.
Active: She will have finished her work.
Passive: Her work will have been finished by her.
Active: They will have elected him their leader.
Passive: He will have been elected their leader (by them).

Changing an interrogative sentence into the passive

Active: Will she have cooked the food?
Passive: Will the food have been cooked by her?
Active: Will they have received our letter?
Passive: Will our letter have been received by them?
Active: Will they have caught the train?
Passive: Will the train have been caught by them?
Active: Who will have opposed you?
Passive: By whom will you have been opposed?

Notes

Active sentences in the future continuous and future perfect continuous tenses cannot be changed to the passive.