Read the passive sentence and find an appropriate subject for the new sentence if there is one. A typical passive voice sentence is: "The window was broken by Tommy Jones." "Window" is the subject of the passive sentence, "was broken" is the verb, and "Tommy Jones" is the agent. The agent is the word which comes after "by." This agent, if there is one, is the best subject for an active voice sentence since the agent is the doer. The doer of the verb is the subject of an active sentence.
Make up an appropriate subject if there is no doer or agent. An example of such a passive voice sentence is: "The window was broken yesterday afternoon." There is no agent or doer mentioned, so we have to invent a possible subject, such as "Somebody" or "Someone."
Take the new subject and place it before the active form of the verb in the original sentence. The passive verb always has some form of "to be" plus the past participle of another verb. The past participle shows the main verb to use in the new sentence. However, put it in the same tense as "be" was in the original sentence and drop the form of "be." For example, "was broken" (with past of "be") becomes "broke" (past of "to break"} in an active voice sentence. Just drop the "was."
Make the old subject of the passive sentence the object of the verb and place it after the verb. Now we have subject plus verb plus object, an active sentence. The first example becomes: "Tommy Jones broke the window." The second becomes "Somebody broke the window yesterday."
Keep the sentence in passive however, if you don't want to mention the doer or call attention to him. Also keep it in passive if you want to put the emphasis on the thing or person who received the action. An example is, "I was mugged last week." That sentence is actually stronger than, "Somebody mugged me last week" if you want to emphasize your plight rather than the unknown mugger. Once you learn how to change passive to active voice by finding the new subject and making the original subject the object, you will be able to choose how to phrase your sentences to give them the emphasis you desire.