Converting Passive to Active:
Make sure the subject of your sentence is the perpetrator of the action, not the recipient.
You can watch out for to-be verbs (was, am, are, etc.) As they can often indicate the use of passive voice, though this isn’t always the case!
Passive: The TARDIS was ridden by the Doctor.
Active: The Doctor rode the TARDIS.
Also Active: The Doctor was riding the TARDIS.
Passive: The Doctor was upset by the color of his kidneys.
Active: The color of his kidneys upset the Doctor.
Passive: Amy was frightened by the Weeping Angels.
Active: The Weeping Angels frightened Amy.
Passive: Rose was left in another universe by the Doctor.
Active: The Doctor left Rose in another universe.
Passive: Clara was dubbed “The Impossible Girl.”
Active: The Doctor dubbed Clara “The Impossible Girl.”
Passive: Young Time Lords are exposed to the Time Vortex.
Active: Elders expose young Time Lords to the Time Vortex.
Do you see in that last example how blame is removed, but the event is still made? It creates distance from the details of the event, yet if a Time Lord elder was telling the story, he might prefer the passive voice because it doesn’t mention who’s at fault.
In scientific writing, passive voice is actually the standard because it creates a feeling of objectivity.
Passive: In a study that was conducted at Harvard, the results showed drastic improvement.
Active: We conducted a study at Harvard which showed drastic improvement.
The passive voice leaves the scientists out of it, which is preferred in that setting. (See that use of passive voice there??)
In fiction, though, active voice should comprise 85-90% of your writing. Passive voice has its place, of course, but you should always be using intentionally.