After you pay the biometrics fee (during your application submission), you will receive a letter asking you to schedule yours, and with instructions on how to.
The letter will have the details, but in a summar, you will need to find a collection service point near you and schedule an appointment.
IRCC website has the list of collection points. Chose one close to you and use their website to schedule your appointment.
Remember to take the request for biometrics (letter) with you on your appointment.