First things first. Brake pads leave a deposit on the metal rotors which actually helps braking power. When the pads or rotors are new, you need to spend 10 minutes carefully bedding in the brakes to ensure that the deposit goes on evenly across the rotors. Otherwise, you’ll end up with an uneven, baked-on, vibration-inducing deposit across the rotors and worst-case you’ll need to replace them.
In a safe area, ride at about 10mph then evenly apply your brakes while seated, until you slow to walking pace. Don’t skid, and vitally, don’t brake to a standstill or you risk leaving a ridge of brake deposit on the rotor. Repeat this 10 times.
Then repeat the process from about 15mph.
You may need more speed or more repetitions for larger rotors, and throughout the process you should feel the braking power increasing.
Building up heat progressively is key here. It might be tempting to build heat by simply dragging your brakes while pedaling, but avoid that, as the wheels won’t be turning evenly enough.