Answer:
Their favorite meals are being cooked by them in the garden now.
Step-by-step explanation:
First, identify the direct subject of this sentence, which is they. Remember to always change things in the independent clause. Then place the indirect object of the sentence at the beginning, which is favorite meals in this case. Switch things around until they make the same meaning (except wordier), and voila - you are done.
Note: most of the time, passive voice is wordy, unclear, and hard to read, so stick with active voice for other writing.
Hope this helps! :)