If you want to change a passive-voice sentence to active voice, find the agent in a "by the..." phrase, or consider carefully who or what is performing the action expressed in the verb. Make that agent the subject of the sentence, and change the verb accordingly. Sometimes you will need to infer the agent from the surrounding sentences which provide context.
If you want to change an active-voice sentence to passive voice, consider carefully who or what is performing the action expressed in the verb, and then make that agent the object of a by the...phrase. Make what is acted upon the subject of the sentence, and change the verb to a form of be + past participle. Including an explicit by the... phrase is optional.
