You need to remove the old one. For that you need to put your car on stands, remove front wheels. Remove the sway bar link. Drain the steering fluid out of the system. Disconnect the steering column...
If the car has an airbag in the steering wheel then take it to a professional mechanic to have it done. If you try to do it yourself then it may come out and cause serious damage. Trust me.
Before you remove the old pulley, it's best to note its position on the pump shaft, and its relation to the other pulleys. The simplest way to install the new pulley (or reinstall the old pulley) is...