What should be done before installing a new plumbing fixture?

Before installing a new plumbing fixture, it is essential to shut off the water supply. This step ensures that there is no water flow during the installation process, which helps prevent leaks, flooding, and water damage. Shutting off the water allows the plumber to work safely and more efficiently without the risk of water spraying or spilling during the removal of existing fixtures or the installation of new ones.

The other actions listed may be helpful in different plumbing contexts but are not prerequisites for fixture installation. Opening all faucets might help in draining existing water from the lines, but it is not the first step to ensure safety. Flushing existing fixtures and running a drain snake address different plumbing concerns, such as clearing clogs or ensuring cleanliness but do not relate directly to the initial actions necessary when installing a new fixture.