Use the following idea to write a sentence in the passive voice .... "save soap and waste paper"
The passive voice occurs in English when the subject of a sentence receives the action as performed by the verb. The passive voice is formed by using an auxiliary or helping verb, some form of "to be" or "to get".
In your example, you can make a sentence in the passive voice as follows:
If soap has been saved, paper will not have been wasted.
There are many other ways this could be written in the passive voice as well, but this is just one example.
For some explanations on the use of the passive voice in English, see the link below.