You use a doorbell transformer to convert your standard household electricity to a lower voltage to operate a doorbell that does not have an internal transformer. Some newer doorbells have the transformer installed inside and you don't need to use an external one. And some newer wireless doorbells work on batteries. To use a doorbell transformer you need basic electrical wiring skills and must know how to select and safely install basic wiring hardware.
Select a location for your doorbell transformer where you can plug it into a wall socket and also run wires to your doorbell and the doorbell button.
Install a standard electric plug on the black and white wires coming out of you doorbell transformer. Use a two prong plug of a style of your choice.
Run wires from your doorbell transformer to the doorbell and the button. Your doorbell transformer has two screw connections on the opposite side from the black and white wires. One connection will go to the doorbell and the other to the button.
Run a wire from the doorbell to the button. Each has two connections. One connection on each is run to the transformer. The other two are connected to each other.
Plug in the transformer and then go press the doorbell button.
Be sure your doorbell transformer has the right voltage output. Usually 10 volts is for buzzers and bells and 16 volts is for chimes. Check your doorbell to be sure. Turn off the power before starting this project.