Set the bicycle on its kickstand, in a bicycle work stand or upside down on the handlebars and seat.
Use a hex wrench, usually a 4mm wrench, to loosen the brake pads on one of your brakes. Do not remove the nuts. Just loosen enough that the brake pads will spin and move up and down in the mounts.
Squeeze the brake lever lightly with one hand to close the brake and move the brake pads toward the rim. While holding the brake lever closed with one hand, use the other to line up the brake pads with the rim. The pads should contact the rim only on the braking surface and should sit flat across it.
Close the brake lever tightly when the brake pads are in place to hold the pads in position. With the other hand, use a hex wrench to tighten the brake pad nuts, making the brake pad's new position permanent.
Loosen the brake mounting bolt, found behind the mounting bracket of the frame or fork, with a hex wrench so that the brake swivels back and forth in the mount. Do not remove the bolt.
Squeeze the brake lever tightly with one hand while tightening the mounting bolt with a hex wrench. This will center the brake so that the pads contact the rim at exactly the same time.
Loosen the cable stop on the left side of the brake with a hex key so that the cable moves freely through it. With one hand, close the brake manually by pushing the brake pads toward the rim. At the same time, pull the cable taught, and then tighten the cable stop so that the cable does not slip or otherwise move.
Pull the brake lever tightly with both hands. Your brake cables, over time, stretch a minute amount, but it is enough to make a difference in braking power. Rather than waiting for this to happen, pull the brake lever as tightly as possible to stretch the cable manually. The brake pads should now move away from the rim slightly so that they clear the rim but still provide adequate braking power.